openstack_net hook: search for ovs_vsctl only when needed

Change-Id: I397e17782f61a94f734cf060d5ecad768379abcc
Bug-Url: https://bugzilla.redhat.com/1077283
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25889
Reviewed-by: Moti Asayag <masayag@redhat.com>
Tested-by: Martin Pavlik <xpavli44@gmail.com>
(cherry picked from commit 89362adee054696d41377a4d8ad6cf993ced91a9)
Reviewed-on: http://gerrit.ovirt.org/25900
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>