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
Merge "BigSwitch: correct net to backend on floating IP disassociation"