Make the Horizon integration tests job voting

Now that it has been running without issues for a few weeks.

Change-Id: Id831774fff4088ad5fe7beb6c3af3fb7028e1632
Closes-Bug: #1355186

zuul/layout.yaml

@@ -844,7 +844,6 @@ jobs:
     voting: false
   - name: gate-horizon-dsvm-integration
     branch: ^(?!stable/(icehouse|juno)).*$
-    voting: false
   # TODO(cschwede): Enable voting when there is enough data
   - name: check-swiftclient-dsvm-functional
     voting: false
@@ -1149,6 +1148,7 @@ projects:
     gate:
       - gate-horizon-python27-django14
       - gate-horizon-selenium
+      - gate-horizon-dsvm-integration
     post:
       - horizon-coverage
     experimental: