Internal server error in gui when subcloud goes down

- Adding checks to prevent user navigating to an offline subcloud
- Adding a default service region when in DC mode to remove the
  possibility of a fresh user being defaulted to logging in to an
  offline subcloud

Change-Id: Ia9d37ad1f977df7e08377677186cd59eae336b1c
Closes-bug: 1890715
Signed-off-by: Tyler Smith <tyler.smith@windriver.com>

starlingx-dashboard/starlingx-dashboard/starlingx_dashboard/dashboards/dc_admin/static/dashboard/dc_admin/cloud_overview/table/subcloud_table.controller.js

@@ -378,6 +378,11 @@
             gettext('The subcloud must be in the managed state before you can access detailed views.'));
         return;
       }
+      if (cloud.availability_status != 'online') {
+        toast.add('error',
+            gettext('The subcloud must be online before you can access detailed views.'));
+        return;
+      }
 
       keystone.getCurrentUserSession().success(function(session){
         session.available_services_regions.indexOf(cloud.name)
@@ -400,6 +405,11 @@
             gettext('The subcloud must be in the managed management state before you can access detailed views.'));
         return;
       }
+      if (cloud.availability_status != 'online') {
+        toast.add('error',
+            gettext('The subcloud must be online before you can access detailed views.'));
+        return;
+      }
 
       keystone.getCurrentUserSession().success(function(session){
         if (session.available_services_regions.indexOf(cloud.name) > -1) {

starlingx-dashboard/starlingx-dashboard/starlingx_dashboard/local/local_settings.d/_30_stx_local_settings.py

@@ -69,6 +69,10 @@ DC_MODE = False
 if distributed_cloud_role and distributed_cloud_role in ['systemcontroller',
                                                          'subcloud']:
     DC_MODE = True
+    DEFAULT_SERVICE_REGIONS = {
+        '*': 'SystemController',
+    }
+
 
 HORIZON_CONFIG["user_home"] = \
     "starlingx_dashboard.utils.settings.get_user_home"