Delete disassociated floating ips on external network deletion

Also remove redundant check for network in use in bigswitch plugin

Closes-Bug: #1238439
Closes-Bug: #1259144
(cherry picked from commit 1763c80711993c55f4f13afe56f449b1dd6d3d3a)

Change-Id: I7586f43c2e99be9df491c68bf1e8658994ffd263
Merge "Delete disassociated floating ips on external network deletion" into stable/icehouse