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>