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/defaults/main.yml

@@ -1,2 +1,7 @@
 wal_g_pg_backup_timer_schedule: >-
   *-*-* 03:56
+
+wal_g_pg_delete_timer_schedule: >-
+  weekly
+wal_g_pg_delete_args: >-
+  retain FIND_FULL 7