Allow mirror script to run on a specified branch.

* jeepyb/cmd/run_mirror.py(Mirror.__init__,Mirror.build_mirror): Add
a -b/--branch option which, if specified, limits the mirror update
to only the named branch rather than running for all available
branches.

Change-Id: I07e5fa98054c8b77bf512166e730d3e73c27286c
Reviewed-on: https://review.openstack.org/28346
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins