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
Would it be possible to use mock.path instead of mock.patch.object? It should just be something like:
with mock.patch("pyleus.cli.topologies.StormCluster", autospec=True) as mock_cluster:
       list_topologies(configs)
 mock_cluster.assert_called_once_with(
       configs.storm_cmd_path,
       configs.nimbus_host,
       configs.nimbus_port,
       configs.verbose,
       configs.jvm_opts,
)
mock_cluster.return_value.list.assert_called_once_with()