vdsm.spec: vdsm-python requires python-cpopen

vdsm/utils.py requires python-cpopen

Change-Id: I32ff1b79148db31705196e631f40d0222138af47
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23451
Reviewed-by: Kiril Nesenko <knesenko@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>