Add horizon to required-projects

The change adds openstack/horizon to required-projects for
murano-dashboard jobs.

Change-Id: I32891d417d4d4bda9c1a810ed3203453d8986af5

zuul.d/projects.yaml

@@ -8583,20 +8583,24 @@
       - system-required
       - check-requirements
       - release-notes-jobs
-      - openstack-python-jobs
-      - openstack-python35-jobs
+      - openstack-python-jobs-horizon
+      - openstack-python35-jobs-horizon
       - translation-jobs
       - translation-jobs-pike
       - translation-jobs-queens
       - nodejs4-jobs
-      - publish-to-pypi
+      - publish-to-pypi-horizon
     check:
       jobs:
         - openstack-tox-cover:
             voting: false
+            required-projects:
+              - openstack/horizon
     post:
       jobs:
-        - openstack-tox-cover
+        - openstack-tox-cover:
+            required-projects:
+              - openstack/horizon
     experimental:
       jobs:
         - legacy-dashboard-dsvm-selenium-package