Fix cached-images API endpoint

The recent chages that renamed 'id' to 'image_id' in the cached_images controller broke the API. This reverts a couple
of those changes.

Change-Id: I83345f87c7e5f1e35ef6646cb360819d489ec256

glance/api/cached_images.py

@@ -54,8 +54,8 @@ def index(self, req):
 
         return dict(cached_images=entries)
 
-    def delete(self, req, image_id):
-        self.cache.purge(image_id)
+    def delete(self, req, id):
+        self.cache.purge(id)
 
     def delete_collection(self, req):
         """
@@ -74,9 +74,9 @@ def delete_collection(self, req):
             num_purged = self.cache.clear()
             return dict(num_purged=num_purged)
 
-    def update(self, req, image_id):
+    def update(self, req, id):
         """PUT /cached_images/1 is used to prefetch an image into the cache"""
-        image_meta = self.get_active_image_meta_or_404(req, image_id)
+        image_meta = self.get_active_image_meta_or_404(req, id)
         try:
             self.cache.queue_prefetch(image_meta)
         except exception.Invalid, e: