Environment Variables for DB creation

[rico-ci]

Following issue #40, there should be the out-of-the-box possibility for the user to define environment variables which should be used for DB creation.

For example, in the CaaS project, I manually added a string parser to substitute keywords starting with $ in the config file with the corresponding environment variables.

I'm not sure whether that's something that should be implemented here or directly in CaaS, but definitely necessary for production-grade deployment.

[rico-ci]

Closed in favour of this issue in CaaS.