Manage AVAILABLE_THEMES with Mirantis theme file

Change-Id: Ib716862fa7163191d41b16bbacca9068be91b7ed
diff --git a/horizon/server/plugin/theme.yml b/horizon/server/plugin/theme.yml
index 85475d7..cf83739 100644
--- a/horizon/server/plugin/theme.yml
+++ b/horizon/server/plugin/theme.yml
@@ -10,3 +10,12 @@
           source:
             engine: pkg
             name: openstack-dashboard-${_param:horizon_dashboard_theme}-theme
+      themes:
+        default: "mirantis"
+        available:
+          mirantis:
+            name: "Mirantis"
+            description: "Mirantis theme"
+            enabled: True
+          material:
+            enabled: False