Mention --fixed-ip subnet_id=<..> in help message

Improved help message for port-create --fixed-ip subnet_id=<..> case:
 * ensure it's mentioned when no arguments are passed;
 * fixed wording of --help message to make it clear subnet_id may be
   passed without ip_address.

Change-Id: Ie48099e0445f80dbff78766e42bc548f925cd208
Closes-Bug: #1267465
Merge "Mention --fixed-ip subnet_id=<..> in help message"