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

Related-Id: Ibaa7ccf8613ee3cce4cb6a72e3206a2c94122222
Change-Id: I6ce09c764436da52ed0a5219c33ae0fb542dd3f8
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
(cherry picked from commit a50bfbf490fd354d08abd25b67aaab83b2a17a85)