Fixing new caching related issue with host vars

The vars_cache was not being properly merged with the setup_cache
for all hosts, which was previously not noticed when registered
variables were stored in the setup_cache.

Fixes #8944

lib/ansible/cache/memcached.py

@@ -186,3 +186,6 @@ class CacheModule(BaseCacheModule):
     def flush(self):
         for key in self.keys():
             self.delete(key)
+
+    def copy(self):
+        return self._keys.copy()