installer: Write PostgreSQL version based on puppet classes.

Using `/etc/init.d/postgresql` as the detection of if Postgres is on
the server is incorrect, because this line runs _before_ puppet and
any packages are installed.  Thus, it cannot tell the difference
between a new Ubuntu one-host first-time-install without PostgreSQL
yet, and one which is merely a front-end and will never have
PostgreSQL.  This leads to failures in first-time installs:

```
Error: Evaluation Error: Error while evaluating a Function Call,
  Could not find template 'zulip/postgresql//postgresql.conf.template.erb'
```

The only way to detect if PostgreSQL will be present in the _end_
state of the install is to examine the puppet classes that are
applied.

To do this, we must inspect `PUPPET_CLASSES`.  Unfortunately, this can
be fragile to subclassing (e.g. `zulip_ops::postgres_appdb`).  We
might desire to use `puppet apply --write-catalog-summary` to deduce
the _applied_ classes, which would unroll the inheritance; however,
this causes a chicken-and-egg problem, because `zulip.conf` must be
already written out (including a value for `postgresql.version`, if
necessary!) before such a puppet run could successfully complete.

Switch to predicating the `postgresql.version` key on the puppet
classes that are known to install postgres.

scripts/lib/install

@@ -329,15 +329,17 @@ auto_renew = yes
 EOF
         fi
 
-        if [ -e "/etc/init.d/postgresql" ]; then
-            if [ "$package_system" = apt ]; then
-                cat <<EOF
+        case ",$PUPPET_CLASSES," in
+            *,zulip::voyager,* | *,zulip::postgres_appdb_tuned,*)
+                if [ "$package_system" = apt ]; then
+                    cat <<EOF
 
 [postgresql]
 version = 12
 EOF
-            fi
-        fi
+                fi
+                ;;
+        esac
     ) > /etc/zulip/zulip.conf
 fi