Resolve env vars into config file

[dvarrazzo]

The config file defines what and how to replicate, which arguably are part of the project where replisome is used. However the specific connection entry points of the database may change, e.g. different deployments, testing environment...

It would be useful if the config file could define some variables to take from the environment: for instance any string starting with $ may be resolved from the env and an error should be raised if not available.