separate cbc proxy into separate clients

this is a pretty big and convoluted refactor unfortunately.

Previously:

There was one global `cbc_proxy_client` object in apps. This class has
the information about how to invoke the bt-ee lambda, and handles all
calls to lambda. This includes calls to the canary too (which is a
separate lambda).

The future:

There's one global `cbc_proxy_client`. This knows about the different
provider functions and lambdas, and you'll need to ask this client for a
proxy for your chosen provider. call cbc_proxy_client.get_proxy('ee')`
and it'll return you a proxy that knows what ee's lambda function is,
how to transform any content in a way that is exclusive to ee, and in
future how to parse any response from ee.

The present:

I also cleaned up some duplicate tests.
I'm really not sure about the names of some of these variables - in
particular `cbc_proxy_client` isn't a client - it's more of a java style
factory, where you call a function on it to get the client of your
choice.

app/clients/cbc_proxy.py

@@ -1,6 +1,7 @@
 import json
 
 import boto3
+from flask import current_app
 
 from app.config import BroadcastProvider
 
@@ -16,9 +17,8 @@ from app.config import BroadcastProvider
 # * description is a string which populates the areaDesc field
 # * polygon is a list of lat/long pairs
 #
-# previous_provider_messages is a whitespace separated list of message identifiers
-# where each identifier is a previous sent message
-# ie a Cancel message would have a unique identifier but have the identifier of
+# previous_provider_messages is a list of previous events (models.py::BroadcastProviderMessage)
+# ie a Cancel message would have a unique event but have the event of
 #    the preceeding Alert message in the previous_provider_messages field
 
 
@@ -26,11 +26,31 @@ class CBCProxyException(Exception):
     pass
 
 
-# Noop = no operation
-class CBCProxyNoopClient:
+class CBCProxyClient:
+    _lambda_client = None
 
     def init_app(self, app):
-        pass
+        if app.config.get('CBC_PROXY_AWS_ACCESS_KEY_ID'):
+            self._lambda_client = boto3.client(
+                'lambda',
+                region_name='eu-west-2',
+                aws_access_key_id=app.config['CBC_PROXY_AWS_ACCESS_KEY_ID'],
+                aws_secret_access_key=app.config['CBC_PROXY_AWS_SECRET_ACCESS_KEY'],
+            )
+
+    def get_proxy(self, provider):
+        proxy_classes = {
+            'canary': CBCProxyCanary,
+            BroadcastProvider.EE: CBCProxyEE,
+        }
+        return proxy_classes[provider](self._lambda_client)
+
+
+class CBCProxyClientBase:
+    lambda_name = None
+
+    def __init__(self, lambda_client):
+        self._lambda_client = lambda_client
 
     def send_canary(
         self,
@@ -67,25 +87,17 @@ class CBCProxyNoopClient:
     ):
         pass
 
+    def _invoke_lambda(self, payload):
+        if not self.lambda_name:
+            current_app.logger.warning(
+                '{self.__class__.__name__} tried to send {payload} but cbc proxy aws env vars not set'
+            )
+            return
 
-class CBCProxyClient:
-    provider_function_name_map = {
-        BroadcastProvider.EE: 'bt-ee-1-proxy',
-    }
-
-    def init_app(self, app):
-        self._lambda_client = boto3.client(
-            'lambda',
-            region_name='eu-west-2',
-            aws_access_key_id=app.config['CBC_PROXY_AWS_ACCESS_KEY_ID'],
-            aws_secret_access_key=app.config['CBC_PROXY_AWS_SECRET_ACCESS_KEY'],
-        )
-
-    def _invoke_lambda(self, function_name, payload):
         payload_bytes = bytes(json.dumps(payload), encoding='utf8')
 
         result = self._lambda_client.invoke(
-            FunctionName=function_name,
+            FunctionName=self.lambda_name,
             InvocationType='RequestResponse',
             Payload=payload_bytes,
         )
@@ -98,15 +110,23 @@ class CBCProxyClient:
 
         return result
 
+
+class CBCProxyCanary(CBCProxyClientBase):
+    """
+    The canary is a lambda which tests notify's connectivity to the Cell Broadcast AWS infrastructure. It calls the
+    canary, a specific lambda that does not open a vpn or connect to a provider but just responds from within AWS.
+    """
+    lambda_name = 'canary'
+
     def send_canary(
         self,
         identifier,
     ):
-        """
-        canary - a specific lambda that does not connect to a provider, but just confirms the connectivity between
-        Notify and the CBC proxy AWS account
-        """
-        self._invoke_lambda(function_name='canary', payload={'identifier': identifier})
+        self._invoke_lambda(payload={'identifier': identifier})
+
+
+class CBCProxyEE(CBCProxyClientBase):
+    lambda_name = 'bt-ee-1-proxy'
 
     def send_link_test(
         self,
@@ -118,7 +138,7 @@ class CBCProxyClient:
         """
         payload = {'message_type': 'test', 'identifier': identifier}
 
-        self._invoke_lambda(function_name='bt-ee-1-proxy', payload=payload)
+        self._invoke_lambda(payload=payload)
 
     def create_and_send_broadcast(
         self,
@@ -134,20 +154,4 @@ class CBCProxyClient:
             'sent': sent,
             'expires': expires,
         }
-        self._invoke_lambda(function_name='bt-ee-1-proxy', payload=payload)
-
-    # We have not implementated updating a broadcast
-    def update_and_send_broadcast(
-        self,
-        identifier, previous_provider_messages, headline, description, areas,
-        sent, expires,
-    ):
-        pass
-
-    # We have not implemented cancelling a broadcast
-    def cancel_broadcast(
-        self,
-        identifier, previous_provider_messages, headline, description, areas,
-        sent, expires,
-    ):
-        pass
+        self._invoke_lambda(payload=payload)