Make image_cache_max_size user configurable

image_cache_max_size should be less than the size of the container. It defaults
to 10GiB which is greater than the current default container size for glance of
5GB.

Change-Id: I58ad98ba3cf83a63dbe346659148aba53dafd140
Related-Bug: 1403487
Merge "Make image_cache_max_size user configurable"