Allow unsharing a network used as gateway/floatingip

If network has ports that connect it as gateway to another network, such
as an external network, or "floating ip" ports it should still be
possible to "unshare" it since these ports represent usage of the
network as an external network, not as a VM data network.

Change-Id: Ic33ae7ac193e4fc9fe06a4cfd579a4aacf0e6354
Closes-bug: #1293184
(cherry picked from commit 7a2053c7a27bdec02e97b46736c5fcce5f2d053a)
Merge "Allow unsharing a network used as gateway/floatingip" into stable/icehouse