media cache: Add CLI commands

This patch adds five CLI commands for the media cache:
 * 'media cache show all' - display a summary of all items in the media
   cache.
 * 'media cache show <uri>' - display detailed information about a
   single item in the media cache.
 * 'media cache delete <uri>' - remove an item from the media cache, and
   inform the bucket backend for the URI scheme to remove the item as
   well.
 * 'media cache refresh <uri>' - refresh a URI. If the item does not
   exist in the media cache, the bucket backend will pull down the media
   associated with the URI and create the item in the cache.
 * 'media cache create <uri>' - create an item in the media cache from
   some local media storage. Note that the bucket backend for the URI
   scheme must still permit the item creation.

Change-Id: Id1c5707a3b8e2d96b56e4691a46a936cd171f4ae