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>