Handle unused set_context in L3NatTestCaseMixin.floatingip_with_assoc

set_context which is passed to floatingip_with_assoc method
is not passed further to self._make_floatingip.

Change-Id: Iecf2ad88e4bad5b1f8fd60668401863bdeecce8f
Closes-Bug: #1378756
Merge "Handle unused set_context in L3NatTestCaseMixin.floatingip_with_assoc"