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
Merge "Make port-id and net-id keys mutually exclusive"