check for prod database before dropping cluster (team#41388)

        export LC_ALL=C.UTF-8 &&

        printf "about to stop and destroy cluster main on postgresql-15, press enter to continue" &&

        read _ &&

        port15=$(grep ^port /etc/postgresql/15/main/postgresql.conf  | sed 's/port.*= //;s/[[:space:]].*$//')

        if psql -P $port15 --no-align --tuples-only \

               -c "SELECT datname FROM pg_database WHERE datistemplate = false and datname != 'postgres';"  \

               | grep .; then

            echo "ERROR: database cluster 15 not empty"

        else

            pg_dropcluster --stop 15 main &&

            pg_upgradecluster -m upgrade -k 13 main &&

            for cluster in `ls /etc/postgresql/13/`; do

                mv /etc/postgresql/13/$cluster/conf.d/* /etc/postgresql/15/$cluster/conf.d/

            done

        fi

    Yes, that implies DESTROYING the *NEW* version but the point is we

    then recreate it with the old one.

    then recreate it from the old one.

    TODO: this whole procedure needs to be moved into fabric, for

    sanity.

 4. change the cluster target in the backup system, in `tor-puppet`,

    for example: