Rework metadata hierarchy for authentication parameters

Since LDAP authentication is supported, it makes sense to have a common
key 'host'.

https://github.com/Mirantis/mk-lab-salt-model/pull/182
diff --git a/nagios/server/init.yml b/nagios/server/init.yml
index a8da37f..b5fea3b 100644
--- a/nagios/server/init.yml
+++ b/nagios/server/init.yml
@@ -26,9 +26,10 @@
       ui:
         enabled: true
         port: ${_param:nagios_ui_port}
-        basic_auth:
-          username: ${_param:nagios_username}
-          password: ${_param:nagios_password}
+        auth:
+          basic:
+            username: ${_param:nagios_username}
+            password: ${_param:nagios_password}
         wsgi:
           port: ${_param:nagios_status_port}
       notification: