Allow specifying the dev database details from an ENV variable
Allow specifying the dev database details from an ENV variable

This is so that publishing end to end tests can specify the name of
the docker container running the database.  By using an environment
variable for the database connection details we also make Signon
more "Twelve Factor".

alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2,alphagov/signonotron2