Use the same bearer token for all API authentication

This removes the `panopticon_api_credentials.rb`
initializer and instead uses the bearer token for
the asset manager as the credentials for the 
Panopticon API.

This also changes the name of the environment var
defining credentials to reflect that these credentials
are no longer used solely to the asset manager.
Merge pull request #128 from alphagov/consolidate-api-credentials

Use the same bearer token for all API authentication