Midonet to support port association at floating IP creation

The Midonet plugin currently does not support associating floating IPs with
ports at floating IP creation time. This bug is created to add this support.

Change-Id: Ie57ebffa5185f26138c04b9836067417c6dc1388
Closes-Bug: #1249957
Merge "Midonet to support port association at floating IP creation"