Change dogpile cache defaults

Memory cache can grow unbounded, so it should really be opt in. Change
to match shade with the following defaults:
 - If you specify nothing, you get null cache
 - If you specify an expiration time and nothing else, you get memory
   cache
 - If you specify an explicit cache class, you will get that class

Change-Id: I6c9eab71a88a534de7e52ad2a564450c44aacc1d

README.rst

@@ -112,13 +112,21 @@ Cache Settings
 
 Accessing a cloud is often expensive, so it's quite common to want to do some
 client-side caching of those operations. To facilitate that, os-client-config
-understands a simple set of cache control settings.
+understands passing through cache settings to dogpile.cache, with the following
+behaviors:
+
+* Listing no config settings means you get a null cache.
+* `cache.max_age` and nothing else gets you memory cache.
+* Otherwise, `cache.class` and `cache.arguments` are passed in
 
 ::
 
   cache:
-    path: ~/.cache/openstack
-    max_age: 300
+    class: dogpile.cache.pylibmc
+    max_age: 3600
+    arguments:
+      url:
+        - 127.0.0.1
   clouds:
     mordred:
       cloud: hp

os_client_config/config.py

@@ -88,13 +88,15 @@ class OpenStackConfig(object):
             self.cloud_config = dict(
                 clouds=dict(openstack=dict(self.defaults)))
 
-        self._cache_max_age = 300
+        self._cache_max_age = None
         self._cache_path = CACHE_PATH
-        self._cache_class = 'dogpile.cache.memory'
+        self._cache_class = 'dogpile.cache.null'
         self._cache_arguments = {}
         if 'cache' in self.cloud_config:
             self._cache_max_age = self.cloud_config['cache'].get(
                 'max_age', self._cache_max_age)
+            if self._cache_max_age:
+                self._cache_class = 'dogpile.cache.memory'
             self._cache_path = os.path.expanduser(
                 self.cloud_config['cache'].get('path', self._cache_path))
             self._cache_class = self.cloud_config['cache'].get(