Fix debian postgresql service name

As we can see below we can't rely on `postgresql.service` to propagate
correctly all actions in all circumstances to `[email protected]`.
It's perhaps due to the fact that `postgresql.service` is configured
with (see `systemctl cat postgresql.service` for more details):
```
[Service]
Type=oneshot
ExecStart=/bin/true
ExecReload=/bin/true
RemainAfterExit=on
```

So it's safer to target explicitely the final service
name (e.g.:`[email protected]`).

Case not OK:
```
systemctl is-active postgresql.service
active

systemctl is-active [email protected]
active

systemctl stop [email protected]

systemctl is-active [email protected]
inactive

systemctl is-active postgresql.service
active
```

Case OK:
```
systemctl start [email protected]

systemctl is-active [email protected]
active

systemctl is-active postgresql.service
active

systemctl stop postgresql.service

systemctl is-active postgresql.service
inactive

systemctl is-active [email protected]
inactive
```

vars/Debian.yml

@@ -1,7 +1,7 @@
 ---
 # PostgreSQL vars for Debian based distributions
 
-postgresql_service_name: "postgresql"
+postgresql_service_name: "postgresql@{{ postgresql_version }}-{{ postgresql_cluster_name }}"
 
 postgresql_bin_directory: /usr/bin