NSX: allow multiple networks with same vlan on different phy_net

Previously, the NSX plugin prevented one from creating multiple networks on
the same vlan even if they were being created on different physical_networks.
This patch corrects this issue and allows this to now occur.

Closes-bug: 1367034

Change-Id: I343449648304328dffdd5ba070491e05686ee22d
(cherry picked from commit 6d1f864531d169a54bc239561840176012629316)
Merge "NSX: allow multiple networks with same vlan on different phy_net" into stable/icehouse