Fix cmd usage msg for ironic port-create

The arguments --address and --node_id are mandatory arguments for
ironic port-create, but are listed as optional arguments in usage message.
This change fixes them.

Change-Id: I3a32577140021ce77706c73f8cc21842ea5f0473
Closes-Bug: #1244637
Merge "Fix cmd usage msg for ironic port-create"