Document image cache manager flag for compute nodes

In Folsom, image cache manager is disabled by default for compute nodes.
It's related to this bug : https://bugs.launchpad.net/nova/+bug/1078594 when we use shared storage.

For infrastructures that don't use shared storage, it's very important to change this default from 0 to a defined value.
Otherwise, you could have some issue with disk space on your compute nodes after a while.

Fix bug #1172580

Change-Id: Ib6400d0b0a40cd4eb4911e9deefef14e33f18912