BigSwitch: correct net to backend on floating IP disassociation

The corrects the network that is updated on the backendw when a
floating IP is disassociated. It was incorrectly sending the
tenant's network when the update is to the external network the
floating address belongs to.

Closes-Bug: #1237209
Change-Id: I55211ba3b0062e167843830bb318eb8e46249160
(cherry picked from commit 39b8bddb1124d16eae15f667f8d921e8ddf5701d)
Merge "BigSwitch: correct net to backend on floating IP disassociation" into milestone-proposed