r/wal-g-pg: Schedule weekly delete jobs

WAL-G slows down significantly when too many backups are kept.  We need
to periodically clean up old backups to maintain a reasonable level of
performance, and also keep from wasting space with useless old backups.

roles/wal-g-pg/meta/main.yml

@@ -1,2 +1,3 @@
 dependencies:
 - dch-yum
+- systemd-base