Compress database backups.

Fixes bug 951600.

The update_gerrit_users.py script creates a database backup
everytime it runs. This change compresses the backups as they
are being made and adds a cron job that will remove backups that
are 30 days or older. Current backups will need to be manually
compressed.

Change-Id: Idb90d4af629892f1bc2e3e98f35f7de2ad58d4be
2 files changed