Declare vpn client option in pipelib

Due to some import refactoring, nova.network.manager isn't always
imported when running pipelib commands, so using the cnt_vpn_clients
option fails. A failing test could not be added because our testing
code imports nova.network.manager so the code config option always
exists in testing.

Fixes bug 1069573

Change-Id: I92b702556bfef2610b86a952dd6f9207fbab102a

nova/cloudpipe/pipelib.py

@@ -52,6 +52,7 @@
                help=_('Netmask to push into openvpn config')),
     ]
 
+flags.DECLARE('cnt_vpn_clients', 'nova.network.manager')
 FLAGS = flags.FLAGS
 FLAGS.register_opts(cloudpipe_opts)