NSX plugin: fix floatingip re-association

The NSX plugin does not allow to reassociate a floating IP to
a different internal IP address on the same port where it's
currently associated.
This patch fixes this behaviour and adds a unit test to ensure
re-association on the same port with a different IP is possible.

A few tweaks to the unit test aux functions were necessary to
accomodate the newly introduced unit test.

Change-Id: Iafbc3c54ebc4509ca75155ef138cc6da869df7bd
Closes-Bug: #1278581
Merge "NSX plugin: fix floatingip re-association"