Fix handling of floating IP association info in Nicira plugin

Fix handling of floating IP association info in Nicira plugin

Bug 1223902

This patch fixes the _update_fix_assoc routine in the Nicira plugin,
ensuring the association of a floating IP with a router is removed
when the floating IP is disassociated.
This patch also adds a unit test for validating behaviour when a floating
IP is associated to a port on a different router, which was an
uncovered use case. This new unit test required some refactoring in
test_l3_plugin.py.

Change-Id: Ibfd53a637f4d14a368b0aeff289c15f993df58dd