Stub SES email client to avoid hitting SES during load testing

If we set an environment variable, we can stub out calls to SES and send
them to our own stub app. If the environment variable is not set, things
work as normal.

To be used alongside
https://github.com/alphagov/notifications-email-provider-stub

app/config.py

@@ -345,6 +345,7 @@ class Config(object):
     # these environment vars aren't defined in the manifest so to set them on paas use `cf set-env`
     MMG_URL = os.environ.get("MMG_URL", "https://api.mmg.co.uk/jsonv2a/api.php")
     FIRETEXT_URL = os.environ.get("FIRETEXT_URL", "https://www.firetext.co.uk/api/sendsms/json")
+    SES_STUB_URL = os.environ.get("SES_STUB_URL")
 
     AWS_REGION = 'eu-west-1'