Move horizon test from tempest-horizon to tempest

As disscussed in Wallaby PTG[1], QA and Horizon team
decided to move the horizon dashboard test from tempest-horizon
to Tempest. As next step, we can remove the tempest-horizon
plugin which will ease the maintaince of horizon tempest test.

[1] https://etherpad.opendev.org/p/qa-wallaby-ptg

Change-Id: Id2ced856a41548a0b49e594ee5fed6ed28785f24
diff --git a/zuul.d/integrated-gate.yaml b/zuul.d/integrated-gate.yaml
index 4c1ee5a..27bbf64 100644
--- a/zuul.d/integrated-gate.yaml
+++ b/zuul.d/integrated-gate.yaml
@@ -69,6 +69,8 @@
       Former names for this job where:
         * legacy-tempest-dsvm-py35
         * gate-tempest-dsvm-py35
+    required-projects:
+      - openstack/horizon
     vars:
       tox_envlist: full
       devstack_localrc:
@@ -89,6 +91,8 @@
             network-feature-enabled:
               qos_placement_physnet: public
       devstack_services:
+        # Enbale horizon so that we can run horizon test.
+        horizon: true
         s-account: false
         s-container: false
         s-object: false