Leverage the NSX networks support for transparent VLANS.
NOTE that the feature needs the configuration variable
cfg.CONF.vlan_transparent to be set to True.
(this is in the neutron configuration file)
This is currently only supported with overlay backing networks.
This is supported from NSX 2.2 onwards.
Change-Id: I4195a4fade42f798689ef19e6d6b59209547beaa