NSXP is the OpenStack support for the NSX Policy platform.
This is supported from neutron in the Stein version. This patch
adds Kolla support

This adds a new neutron_plugin_agent type 'vmware_nsxp'. The plugin
does not run any neutron agents.

Change-Id: I9e9d8f07e586bdc143d293e572031368af7f3fca