Make DB host for MaaS configurable

Change-Id: I54f2113ac1d067a327668dda3aee288b9ad0f407
Related-prod: PROD-28242 (PROD:28242)
diff --git a/maas/region/single.yml b/maas/region/single.yml
index 96ba7c6..52a613e 100644
--- a/maas/region/single.yml
+++ b/maas/region/single.yml
@@ -49,7 +49,7 @@
         email:  email@example.com
       database:
         engine: postgresql
-        host: localhost
+        host: ${_param:maas_postgresql_server}
         name: maasdb
         password: ${_param:maas_db_password}
         username: maas