Make port-id and net-id keys mutually exclusive

These keys are never used together, and port-id always takes precedence
in Nova API when present. This patch makes these parameters mutually
exclusive and updates help strings accordingly.

Change-Id: I207dc637cb69d9ab3ff6024a657d2c1890057b4d
Closes-Bug: #1314097
Add extension-list command for v3 API

Nova returns available extension list through its REST API.
The command of v2 API has been implemented, and this patch adds the
one of v3 API.

Note:
The command name of v2 is "nova list-extensions" but the one of v3,
which this patch adds, is "nova extension-list" because the name fits
to the other command names.

Change-Id: Ibee712b0aa5898a94e362f20be71e8863500b195
Merge "Add extension-list command for v3 API"
Merge "Make port-id and net-id keys mutually exclusive"