Move memcached_servers opt into common.memorycache

Add a factory function to nova.common.memorycache which consolidates
the code to choose between real or fake memcache. This then means that
memcached_servers is used in just one place and we can move the config
option there.

blueprint: scope-config-opts
Change-Id: I67c191e0db58364eda4162b9e881606063509b9d

nova/common/memorycache.py

@@ -18,8 +18,28 @@
 
 """Super simple fake memcache client."""
 
+from nova.openstack.common import cfg
 from nova.openstack.common import timeutils
 
+memcache_opts = [
+    cfg.ListOpt('memcached_servers',
+                default=None,
+                help='Memcached servers or None for in process cache.'),
+]
+
+CONF = cfg.CONF
+CONF.register_opts(memcache_opts)
+
+
+def get_client():
+    client_cls = Client
+
+    if CONF.memcached_servers:
+        import memcache
+        client_cls = memcache.Client
+
+    return client_cls(CONF.memcached_servers, debug=0)
+
 
 class Client(object):
     """Replicates a tiny subset of memcached client interface."""