salt: handle edge cases when calling service_configuration salt module

Since the introduction of service_configurations, some charts such as prometheus-adapter
and nginx-ingress-controller do not yet implement this functionality but might in future
have configurable service values.

To render all charts, we mandatorily need to pass arguments for `--service-config` and since the
above charts do not yet implement this feature, we should be able to handle them differently.

Now, we pass `unavailable` as the service configmap name such that the module can handle things
else, if we pass `None` an exception will be raised.

salt/_modules/metalk8s_service_configuration.py

@@ -43,6 +43,12 @@ def get_service_conf(
             'Expected a ConfigMap name but got {}'.format(configmap_name)
         )
 
+    # rendering charts like prometheus-adapter & nginx-ingress-controller
+    # require the user to pass --service-config arguments but these charts
+    # don't yet have service configs, so let us handle this case differently
+    if configmap_name == 'unavailable':
+        return None
+
     try:
         manifest = __salt__[
             'metalk8s_kubernetes.get_object'