improve speed of metadata

* don't load every possible answer, only do what is needed
 * cache instance data for a given address for a 15 seconds
   using either memcache or fake memcache (in-memory).
   This means only a single queue/db lookup for multiple calls
   to metadata service
 * add cache expirey to fake memcache (don't grow forever)
   and move it to nova.common.memorycache

Addresses Bug #851159

Change-Id: Icf794156e055b18915b8b5be9ba2ab97d2338bbe

nova/auth/ldapdriver.py

@@ -96,7 +96,7 @@ LOG = logging.getLogger(__name__)
 if FLAGS.memcached_servers:
     import memcache
 else:
-    from nova.testing.fake import memcache
+    from nova.common import memorycache as memcache
 
 
 # TODO(vish): make an abstract base class with the same public methods

nova/auth/manager.py

@@ -97,7 +97,7 @@ LOG = logging.getLogger(__name__)
 if FLAGS.memcached_servers:
     import memcache
 else:
-    from nova.testing.fake import memcache
+    from nova.common import memorycache as memcache
 
 
 class AuthBase(object):

nova/common/memorycache.py

@@ -29,11 +29,16 @@ class Client(object):
         self.cache = {}
 
     def get(self, key):
-        """Retrieves the value for a key or None."""
-        (timeout, value) = self.cache.get(key, (0, None))
-        if timeout == 0 or utils.utcnow_ts() < timeout:
-            return value
-        return None
+        """Retrieves the value for a key or None.
+
+        this expunges expired keys during each get"""
+
+        for k in self.cache.keys():
+            (timeout, _value) = self.cache[k]
+            if timeout and utils.utcnow_ts() >= timeout:
+                del self.cache[k]
+
+        return self.cache.get(key, (0, None))[1]
 
     def set(self, key, value, time=0, min_compress_len=0):
         """Sets the value for a key."""

nova/testing/fake/__init__.py

@@ -1,2 +1 @@
-import memcache
 import rabbit