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: I61170291468757b47b043bd81f022704dd38e3d0
Closes-Bug: #1314097
Merge "Make port-id and net-id keys mutually exclusive"