NSX: properly handle floating ip status

Ensure the floating IP status is put ACTIVE or DOWN when it
is associated or disassociated.
This is done directly on the plugin class as the NSX backend
applies relevant NAT rules synchronously.

Change-Id: Ie747c4eae2ac33016fdabaade2335f7d2d24eec9
Closes-Bug: #1334708
Merge "NSX: properly handle floating ip status"