VMWare: don't notify on disassociate_floatingips()

L3 agent notifications don't make sense for NSX VMWare plugin since
there is no L3 agent in such setup, so disabling them here.

Updated a unit test to check that notification is indeed not requested.

Change-Id: I9c7c32d02d466098d22df8f10448361c3d99174c
Related-Bug: 1330955
Merge "VMWare: don't notify on disassociate_floatingips()"