Categorize mistral-dashboard into horizon-plugin

mistral-dashboard is the only deliverable which is not marked as
horizon-plugin now. This commit moves mistral-dashboard to separate
deliberable files.

Change-Id: I457f377f841858f591c18ae2c17c0764e523d150

deliverables/ussuri/mistral-dashboard.yaml

@@ -0,0 +1,7 @@
+---
+launchpad: mistral
+release-model: cycle-with-rc
+team: mistral
+type: horizon-plugin
+repository-settings:
+  openstack/mistral-dashboard: {}