mysql: make server config optional

[rwnx]

This is to support usage of local config files (my.cnf) see #108

• I've skipped postgres because i need to investigate more about the pgpass file. I think we still need to pass a lot of config, only the password can be replaced
• I've skipped mssql because we're not using the CLI, so no cli-behaviour can be expected
• Needs: a closer look at an integration test

[rwnx]

There's definitely more context about what options can/should be passed via config file. Maybe we should keep this limited to username/password for now?

https://tomlankhorst.nl/mysql-my-cnf-options-file/

[rwnx]

Postgres support, integration tests have now been added!

[balcsida]

Fixes #108