wal-g was used in `puppet/zulip` by env-wal-g, but only installed in
`puppet/zulip_ops`.
Merge all of the dependencies of doing backups using wal-g (wal-g
installation, the pg_backup_and_purge job, the nagios plugin that
verifies it happens) into a common base class in `puppet/zulip`, since
it is generally useful.