Add nimbus port option

This expands upon #32, so I will close that PR.

In summary, this change:
* Renames `-n/--nimbus-ip/nimbus_ip/NIMBUS_IP` to `-n/--nimbus-host/nimbus_host/NIMBUS_HOST`
* Adds `-p/--nimbus-port/nimbus_port/NIMBUS_PORT`
* Adds some tests of this and other behavior surrounding config vars
I'd like to list here every useful option users can configure from this file, so can you add also `nimbus_port`?