Add admin doc information about image cache resource accounting
Add admin doc information about image cache resource accounting

This adds some details to the image cache page in the admin docs about
how image cache disk usage is (not) considered in the scheduler disk
space calculation. Workarounds and mitigation strategies are provided.

Change-Id: I7f40f167cea073a73cf249a9adfd73e1187c031b
Related-Bug: #1878024

openstack/nova,mahak/nova,openstack/nova,klmitch/nova,klmitch/nova,openstack/nova,mahak/nova,mahak/nova,klmitch/nova,klmitch/nova