Define failover lambdas

We will need a lambda to failover to if the first lambda fails. This
isn't so much a case of the lambda itself failing, as it is a cross
availability zone resource automatically, it's more in case something in
the networking goes down in our AZ and therefore the lambda can't call
out to the CBC. In this case, we will be able to swap to using the
second AZ by calling the second lambda.

app/clients/cbc_proxy.py

@@ -56,6 +56,11 @@ class CBCProxyClientBase(ABC):
     def lambda_name(self):
         pass
 
+    @property
+    @abstractmethod
+    def failover_lambda_name(self):
+        pass
+
     @property
     @abstractmethod
     def LANGUAGE_ENGLISH(self):
@@ -136,6 +141,9 @@ class CBCProxyCanary(CBCProxyClientBase):
     canary, a specific lambda that does not open a vpn or connect to a provider but just responds from within AWS.
     """
     lambda_name = 'canary'
+    # we don't need a failover lambda for the canary as it doesn't actually make calls out to a CBC
+    # so we just reuse the normal one in case of a failover scenario
+    failover_lambda_name = 'canary'
 
     LANGUAGE_ENGLISH = None
     LANGUAGE_WELSH = None
@@ -149,6 +157,7 @@ class CBCProxyCanary(CBCProxyClientBase):
 
 class CBCProxyEE(CBCProxyClientBase):
     lambda_name = 'bt-ee-1-proxy'
+    failover_lambda_name = 'bt-ee-2-proxy'
 
     LANGUAGE_ENGLISH = 'en-GB'
     LANGUAGE_WELSH = 'cy-GB'
@@ -208,6 +217,7 @@ class CBCProxyEE(CBCProxyClientBase):
 
 class CBCProxyVodafone(CBCProxyClientBase):
     lambda_name = 'vodafone-1-proxy'
+    failover_lambda_name = 'vodafone-2-proxy'
 
     LANGUAGE_ENGLISH = 'English'
     LANGUAGE_WELSH = 'Welsh'