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>

configuration/configuration.go

@@ -187,7 +187,12 @@ type Storage map[string]Parameters
 func (storage Storage) Type() string {
 	// Return only key in this map
 	for k := range storage {
-		return k
+		switch k {
+		case "cache":
+			// allow configuration of caching
+		default:
+			return k
+		}
 	}
 	return ""
 }
@@ -211,9 +216,17 @@ func (storage *Storage) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		if len(storageMap) > 1 {
 			types := make([]string, 0, len(storageMap))
 			for k := range storageMap {
-				types = append(types, k)
+				switch k {
+				case "cache":
+					// allow configuration of caching
+				default:
+					types = append(types, k)
+				}
+			}
+
+			if len(types) > 1 {
+				return fmt.Errorf("Must provide exactly one storage type. Provided: %v", types)
 			}
-			return fmt.Errorf("Must provide exactly one storage type. Provided: %v", types)
 		}
 		*storage = storageMap
 		return nil