Fix to delete user and group association in Nuage Plugin

After a router delete operation, the attached zone to that
router is also deleted. Got rid of code that tried to do
a get operation on the nuage_zone after router delete
operation.
Closes-Bug: #1367864

Change-Id: I01753a472200a961cdcecee703616fd3239abd3c
Merge "Fix to delete user and group association in Nuage Plugin"