Add `db-tunnel` options to the configuration file

[benjlevesque]

Context

When using db-tunnel, I would like to fallback to the configuration file value if no --port flag was specified (or any other flag).

Currently, only the region is available in the configuration file, it would be useful to add options for more options.

The use of this feature is tightly linked to #1080.

Proposed solution

{
  "region": "osc-fr1",
  "db-tunnel": {
    "port": 1234,
    "bind": "127.0.0.1"
  }
}

Going further

This is probably a need for most commands, but this is the only one I currently use or need, so I didn't want to make the scope of this issue too wide.