Allow control over which storage cache to use

This allows one to better control the usage of the cache and turn it off
completely. The storage configuration module was modified to allow parameters
to be passed to just the storage implementation, rather than to the driver.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/config.yml

@@ -5,6 +5,8 @@ log:
     service: registry
     environment: development
 storage:
+    cache:
+        layerinfo: inmemory
     filesystem:
         rootdirectory: /tmp/registry-dev
 http: