VMware: add support for simple DVS

Add the file vmware_dvs that will enable the external CI to be
run.

The patch in the VMware repo for the DVS support is:
https://review.openstack.org/#/c/177597/

Change-Id: I6dee978fd2be3818c5ce57b1dcb2917234ab61e2

lib/neutron_plugins/vmware_dvs

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+# This file is needed so Q_PLUGIN=vmware_dvs will work.
+
+# FIXME(salv-orlando): This function should not be here, but unfortunately
+# devstack calls it before the external plugins are fetched
+function has_neutron_plugin_security_group {
+    # 0 means True here
+    return 0
+}