NSX|P: Return False at the end of _is_overlay_net

Currently it returns None as default, but the nicer result is False
if the network is not an overlay one.

Change-Id: Ifb490cd05f76714a63e37b1808a0ff9fd1dfa40a

vmware_nsx/plugins/nsx_p/plugin.py

@@ -3654,6 +3654,8 @@ class NsxPolicyPlugin(nsx_plugin_common.NsxPluginV3Base):
                     tz)
                 return type == nsxlib_consts.TRANSPORT_TYPE_OVERLAY
 
+        return False
+
     def _is_ens_tz(self, tz_id):
         # This call is cached on the nsxlib side
         mode = self.nsxpolicy.transport_zone.get_host_switch_mode(tz_id)