NSX plugin: set VNIC_TYPE port binding attribute

A recent change has made this attribute required for nova
integration.
This patch adds this attribute to responses generated by the NSX
plugin, and also ensures relevant unit tests are executed for the
vmware NSX plugin.

Change-Id: Ieebab01b406909f66a40cc683763292a9ef6f218
Closes-Bug: #1370112
Merge "NSX plugin: set VNIC_TYPE port binding attribute"