Fix 'glance-cache-manage list-cached' for xattr

The xattr driver for the caching middleware was returning it's
last_modified/last_accessed times in iso8601 format rather than a
timestamp.

This was inconsistent with the sqlite driver and caused the
glance-cache-manage command to fall over when using the xattr driver.

Fixes bug 1197360

Change-Id: Ie72dcd9f41f7a3b7405cda9fe28e1e468fcc1bef
Merge "Fix 'glance-cache-manage list-cached' for xattr"