Create Keystone admin endpoint for horizon integartion job

CI job "horizon-integration-tests" needs an admin endpoint. Since
[1] devstack does not create it by default. This patch enables it
in this job now.

[1] https://review.opendev.org/c/openstack/devstack/+/777345

Related-Bug: #1950659
Change-Id: I5a81344ea3aaa182d8b4d6a15a6240d3c99557cf

.zuul.d/tempest-and-integrated.yaml

@@ -24,6 +24,11 @@
       - ^openstack_dashboard/.*/[^/]*\.spec\.js$
       - ^horizon/static/.*/[^/]*\.spec\.js$
     vars:
+      devstack_localrc:
+        # NOTE: workaround for bug 1950659
+        # Some horizon integration test start failing after devstack
+        # stop creating default keystone admin endpoint.
+        KEYSTONE_ADMIN_ENDPOINT: true
       devstack_services:
         horizon: true
       tox_envlist: integration