Adds test for deleting external network with floatingIPs

The attached neutron bug causes server to hang when deleting external network
that still has a floating IP in it.
This test should recreate the bug, and verify it is fixed

Closes-Bug: #1374573

Change-Id: Ib7d8dcbb4485e87a49cb008ace37c81f6b06a32c
Merge "Adds test for deleting external network with floatingIPs"