Set CBC_PROXY_ENABLED per environment, not dynamically

Previously we looked at whether an environment was given AWS access keys
to decide if the `CBC_PROXY_ENABLED` setting was true. Given that all
environments (apart from development) are currently hooked up to our AWS
cell broadcast accounts, it doesn't feel too useful to have a dynamic
switch when we can just hardcode it.

On top of that, this lays the groundwork for having `CBC_PROXY_ENABLED`
to be True even if an individual application doesn't have the CBC PROXY
aws access keys as in future only the broadcasts worker will have the
AWS keys but all the other apps will know that cell broadcasting is
indeed turned on for that environment.

app/config.py

@@ -365,11 +365,10 @@ class Config(object):
 
     AWS_REGION = 'eu-west-1'
 
+    CBC_PROXY_ENABLED = True
     CBC_PROXY_AWS_ACCESS_KEY_ID = os.environ.get('CBC_PROXY_AWS_ACCESS_KEY_ID', '')
     CBC_PROXY_AWS_SECRET_ACCESS_KEY = os.environ.get('CBC_PROXY_AWS_SECRET_ACCESS_KEY', '')
 
-    CBC_PROXY_ENABLED = bool(CBC_PROXY_AWS_ACCESS_KEY_ID)
-
     ENABLED_CBCS = {BroadcastProvider.EE, BroadcastProvider.THREE, BroadcastProvider.O2, BroadcastProvider.VODAFONE}
 
     # as defined in api db migration 0331_add_broadcast_org.py
@@ -420,6 +419,8 @@ class Development(Config):
     API_RATE_LIMIT_ENABLED = True
     DVLA_EMAIL_ADDRESSES = ['success@simulator.amazonses.com']
 
+    CBC_PROXY_ENABLED = False
+
 
 class Test(Development):
     NOTIFY_EMAIL_DOMAIN = 'test.notify.com'