Add memcache_pool backend job

This is to be able to validate in CI that it actually works.

Change-Id: Iadad6d2d4643412e4d846a05c3e3eb4a3deecb84
Related-bug: #1866008

.zuul.yaml

@@ -1,3 +1,13 @@
+- job:
+    name: oslo.cache-tempest-full-py3-memcached-pool
+    parent: tempest-full-py3
+    voting: false
+    description: |
+      Base integration test with memcache_pool backend.
+    vars:
+      devstack_localrc:
+        CACHE_BACKEND: oslo_cache.memcache_pool
+
 - project:
     templates:
       - check-requirements
@@ -7,3 +17,6 @@
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
+    check:
+      jobs:
+        - oslo.cache-tempest-full-py3-memcached-pool