janitorial: drop remaining betterPopen references

betterPopen has been named cpopen long ago, and was spun off to an
independent package. This patch clears the remaining references to the
old vague name.

Change-Id: Idba693ef181f948e0ac4a525326698ef96684318
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22195
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Francesco Romani <fromani@redhat.com>