Allow to override credentials for Commons database via environment variables

In 141efb0, we allowed to override the credentials to connect
to the Commons database through the configuration file.

However, in the use case of proxying through Toolforge via a Docker setup,
we do not want to hardcode these credentials but rather to pass them
through using environment variables.

Change-Id: Ifd7c9abacda5824ff8748ceadf0a39598431760a