Add command line option for output directory.

Add a -o, --out-dir option to specify an output directory.

Change-Id: I0c8ac1aba309f9a34b2562b18da644efd9f37f4e
Reviewed-on: https://review.openstack.org/20839
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Dan Prince <dprince@redhat.com>
Reviewed-by: Dan Prince <dprince@redhat.com>
Tested-by: Jenkins