r/wal-g-pg: Schedule daily backups

WAL archives are not much good without a base backup onto which they
can be applied.  Thus, we need to schedule WAL-G to create and upload a
backup periodically.

roles/wal-g-pg/files/wal-g-backup.service

@@ -0,0 +1,9 @@
+[Unit]
+Description=PostgreSQL Backup with WAL-G
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Type=oneshot
+Environment=PGDATA=/var/lib/pgsql/data
+ExecStart=/usr/bin/wal-g-pg backup-push --config /etc/postgresql/wal-g.yml $PGDATA