Store MySQL credentials in environment

[bcongdon]

It's pretty cumbersome to necessitate a separate MySQL config in each service. We should allow services to pull MySQL credentials (user, pass, host, port) from env variables.

This will make things a lot easier for setting up a dev / prod environment, as well as for setting up Docker.

This change will need to be made in all the services that use MySQL.

[narendasan]

Might be something we would want to include in the groot-deploy tools, however if we decide to change from mysql it gets a bit more complicated