api: add image_cache_dir parameter

Without image_cache_dir parameter, Glance API can't start.
Default value is set to /var/lib/glance/image-cache.

Change-Id: I1cfcbf0021cfae187d9c8d78a16b18de29c97b4a
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>