Allows use of external database or renaming of docker database service.

[ctownsen357]

Fixes #
This allows one to use an external PostgreSQL database rather than requiring the docker ckan container to point to the db service within the compose file. In our case this allows us to point our containers to an AWS RDS instance. This change programmatically determines the database host based on the required CKAN_SQLALCHEMY_URL environment variable rather than leaving it hard coded to the compose service name.

This change also allows the system to fail fast if required environment variables are not set rather than waiting for the database to be available before testing the environment variables.

Proposed fixes:

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

Please [X] all the boxes above that apply