Fix policy failure on image_metadata calls

Image metadata create/update/delete calls were not catching failures
returned by Glance and translating them into nice returns for API
callers.  Now ImageNotAuthorized failures are caught and passed back to
the caller as 403s.

bug 1204646

Change-Id: Ib9e451531d8102b6089e2cfc9be16d1bd635d59d
Merge "Fix policy failure on image_metadata calls"