Add support for using a SOCKS5 proxy to connect to live replica database

[audiodude]

This should be documented somewhere, but for now the steps are:

1. Sign up for a toolforge account.
2. Set up your ~/.ssh/config so you can easily login to toolforge
3. Run a SOCKS5 proxy with ssh -D 0.0.0.0:1080 login.toolforge.org
4. Update default_config.yaml or add a config.yaml in the same directory, with values for the keys:
   • REPLICA_SOCKS5_PROXY_HOST -- The IP or hostname of the Docker host where the proxy is running.
   • REPLICA_SOCKS5_PROXY_PORT -- The port you used for -D above
   • REPLICA_DOMAIN -- Set to: 'analytics.db.svc.wikimedia.cloud'
   • REPLICA_USER -- Your MariaDB credentials, found in replica.my.cnf in your toolforge home directory
   • REPLICA_PASSWORD -- Same as above, but the password

Also this PR removes some values from default_config that aren't referenced anywhere in the code (REPLICA_DOMAIN and REPLICA_HOST)

[siddharthvp]

Neat! Can confirm this works.