vm-builder: Turn off --auto-discover-databases (#571)

It causes exporter connection to every database in the Postgres and it
was disabled for pods a while ago, but I forgot that VMs have a
different build / control path.

See this commit for details:
  neondatabase/cloud@0a60057

Also remove `--exclude-databases` it seems to be bogus as well, see:
  prometheus-community/postgres_exporter#588

neonvm/tools/vm-builder/main.go

@@ -251,7 +251,7 @@ fi
 ::respawn:su -p vm-monitor -c 'RUST_LOG=info /usr/local/bin/vm-monitor --addr "0.0.0.0:10301" --cgroup=neon-postgres{{if .FileCache}} --pgconnstr="host=localhost port=5432 dbname=postgres user=cloud_admin sslmode=disable"{{end}}'
 {{end}}
 ::respawn:su -p nobody -c '/usr/local/bin/pgbouncer /etc/pgbouncer.ini'
-::respawn:su -p nobody -c 'DATA_SOURCE_NAME="user=cloud_admin sslmode=disable dbname=postgres" /bin/postgres_exporter --auto-discover-databases --exclude-databases=template0,template1'
+::respawn:su -p nobody -c 'DATA_SOURCE_NAME="user=cloud_admin sslmode=disable dbname=postgres" /bin/postgres_exporter'
 ttyS0::respawn:/neonvm/bin/agetty --8bits --local-line --noissue --noclear --noreset --host console --login-program /neonvm/bin/login --login-pause --autologin root 115200 ttyS0 linux
 ::shutdown:/neonvm/bin/vmshutdown
 `