Fix the AttributeError: __exit__ error

The memcache client class actually has no __exit__ function.

Remove the "with" usage to avoid the __exit__ error.

Change-Id: I15b3d08f4afae289e7eb0848ff1db08141196d3c
Closes-Bug: #1747565

keystonemiddleware/auth_token/_cache.py

@@ -98,8 +98,7 @@ class _MemcacheClientPool(object):
 
     @contextlib.contextmanager
     def reserve(self):
-        with self._pool.get() as client:
-            yield client
+        yield self._pool.get()
 
 
 class TokenCache(object):

keystonemiddleware/tests/unit/auth_token/test_cache.py

@@ -151,11 +151,17 @@ class TestLiveMemcache(base.BaseAuthTokenTestCase):
         token_cache.set(token, data)
         self.assertEqual(token_cache.get(token), data)
 
-    def test_memcache_pool_init(self):
+    def test_memcache_pool(self):
         conf = {
             'memcached_servers': ','.join(MEMCACHED_SERVERS),
             'memcache_use_advanced_pool': True
         }
 
+        token = six.b(uuid.uuid4().hex)
+        data = uuid.uuid4().hex
+
         token_cache = self.create_simple_middleware(conf=conf)._token_cache
         token_cache.initialize({})
+
+        token_cache.set(token, data)
+        self.assertEqual(token_cache.get(token), data)