Adding 'download_image' policy enforcement to image cache middleware
    
    Currently image cache middleware not care 'download_image' policy, the
    enforcement caused user receive empty content but with HTTP 200 code
    rather than 403 when client attempt to download image using v2 API. And
    the real Forbidden exception be logged in glance-api log which image
    application action raised. The end user is confused by this behavior.
    
    Fixes bug: 1235378
    
    Change-Id: Ibaa7ccf8613ee3cce4cb6a72e3206a2c94122222
    Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>