Adding l3_ext network type support for neutron VMWare NSX plugin

http://docs.openstack.org/admin-guide-cloud/content/section_plugin_specific_extensions.html#section_vmware_nsx_plugin_l3_extension

Change-Id: Ibabe05ebfa6315285c72465da2eadc989b3a9312
Merge "Adding l3_ext network type support for neutron VMWare NSX plugin"