Provide SECRET_KEY env var for apps
Provide SECRET_KEY env var for apps

Currently we use different environment variable names for the Flask
secret key and then override it with os.environ in the root Config
class.

This is problematic for a few reasons:

1. SECRET_KEY is a Flask thing and naming it something different (even
   if it is more descriptive) obfuscates what it's being used for.
2. Setting it from the environment in the base class means that we
   cannot set a default for it in one of the environment specific child
   classes.

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws