Start building a bandersnatch mirror

Bandersnatch is more efficient than pypi-mirror, so we can just
run it in cron on the mirror host. Overlay the two mirrors in
the same dir so that we can transition. Landing this patch will
involve running a manual copy command on the host.

Change-Id: Iad1f3d24016cb8e262d69102ba8847e9dfaa0ca0
Merge "Start building a bandersnatch mirror"