BigSwitch: Fixes floating IP backend updates

Changes BigSwitch plugin to correctly use
admin context on floating IP updates to the
backend controller so they correctly contain
floating IPs for all tenants.

Closes-Bug: #1262488
Change-Id: I6f2666c242e6d9b0684943db073a2284d01fa1e0
Merge "BigSwitch: Fixes floating IP backend updates"