Fixed floating IP logic in PLUMgrid plugin

  * Fixed associate and disassociate
    floating IP logic
  * Added unit tests

Change-Id: I310f5550bca1c9015cbda3e0338eb90a36dc96f1
Closes-Bug: 1298495
(cherry picked from commit 9f8cb33557ecc7c0a832610e913a56a8b8e6c524)
Merge "Fixed floating IP logic in PLUMgrid plugin" into stable/havana