Add zuul pack-refs cron

Ad a cron job to pack refs in zuul repos.  Zuul creates _a lot_
of refs, packing them should make client git repo access more
efficient.  Up to a point at least -- ultimately we may still
need to delete them later.

Change-Id: Iaf5180069b38833d6e25f6880e4868d5e2319b75
Merge "Add zuul pack-refs cron"