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/__init__.py

@@ -23,7 +23,7 @@ from werkzeug.local import LocalProxy
 
 from app.celery.celery import NotifyCelery
 from app.clients import NotificationProviderClients
-from app.clients.cbc_proxy import CBCProxyClient, CBCProxyNoopClient
+from app.clients.cbc_proxy import CBCProxyClient
 from app.clients.document_download import DocumentDownloadClient
 from app.clients.email.aws_ses import AwsSesClient
 from app.clients.email.aws_ses_stub import AwsSesStubClient
@@ -61,7 +61,7 @@ zendesk_client = ZendeskClient()
 statsd_client = StatsdClient()
 redis_store = RedisClient()
 performance_platform_client = PerformancePlatformClient()
-cbc_proxy_client = CBCProxyNoopClient()
+cbc_proxy_client = CBCProxyClient()
 document_download_client = DocumentDownloadClient()
 metrics = GDSMetrics()
 
@@ -114,9 +114,6 @@ def create_app(application):
     performance_platform_client.init_app(application)
     document_download_client.init_app(application)
 
-    global cbc_proxy_client
-    if application.config['CBC_PROXY_AWS_ACCESS_KEY_ID']:
-        cbc_proxy_client = CBCProxyClient()
     cbc_proxy_client.init_app(application)
 
     register_blueprint(application)