Adding VLAN Transparency support for ML2 along with REST API changes

Adding VLAN Transparency support for ML2 along with REST API changes

* Exposing vlan transparency attributes via the network api
  calls so POST and GET operations can be performed.
* Tied in the vlan transparency attribute with create network
  call and use the config default.
* Update the unit test to cover vlan tranparency.
* Add support for ml2 to take advantage of vlan transparency
  attribute.

DocImpact
APIImpact
Partially Implements: blueprint nfv-vlan-trunks

Change-Id: Ie87087a70b83dab589419aa5c17ce7ccafd64cbd