Mirror script output parsing fix.

* jeepyb/cmd/run_mirror.py: Output parsing for pip install
--no-install was looking for "Successfully installed" when it should
have been looking for "Successfully downloaded" instead. This fixes.

Change-Id: Iaee9e95f8cb035a2c1a398cedb62ab3738c5ef35
Reviewed-on: https://review.openstack.org/23469
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins