Adding functional tests for memcache_pool backend

Introducing the needed changes to functional tests to run
them against the memcache_pool backend.

These changes introduced a new zuul job related to memcache
to isolate this part during CI and get clear outputs.

Change-Id: Ib84b3db83e8c50c7929874c63177c94f459a1a2c
Co-authored-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>

.zuul.yaml

@@ -37,6 +37,13 @@
       tox_environment:
           OSLO_BACKEND: dogpile_cache_bmemcached
 
+- job:
+    name: oslo.cache-functional-memcache_pool
+    parent: oslo.cache-functional-memcached
+    vars:
+      tox_environment:
+        OSLO_BACKEND: memcache_pool
+
 - project:
     templates:
       - check-requirements
@@ -50,3 +57,4 @@
       jobs:
         - oslo.cache-functional-etcd3gw
         - oslo.cache-functional-dogpile.cache.bmemcached
+        - oslo.cache-functional-memcache_pool

oslo_cache/tests/functional/memcache_pool/test_cache_backend.py

@@ -0,0 +1,32 @@
+#    Copyright 2020 OpenStack Foundation
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import os
+
+from oslo_cache.tests.functional import test_base
+
+
+class TestMemcachePoolCacheBackend(test_base.BaseTestCaseCacheBackend):
+    def setUp(self):
+        MEMCACHED_PORT = os.getenv("OSLO_CACHE_TEST_MEMCACHED_PORT", "11211")
+        self.config_fixture.config(
+            group='cache',
+            backend='oslo_cache.memcache_pool',
+            enabled=True,
+            memcache_servers=[f'localhost:{MEMCACHED_PORT}']
+        )
+        # NOTE(hberaud): super must be called after all to ensure that
+        # config fixture is properly initialized with value related to
+        # the current backend in use.
+        super(TestMemcachePoolCacheBackend, self).setUp()