Add documentation for configuring disk_formats
Add documentation for configuring disk_formats

Add documentation explaining how operators can add or remove supported
disk_formats.

Change-Id: I14f6c53506590e6fa373afdbdc3b6f052a0a532c
Closes-bug: 1519006

rajalokan/glance,stevelle/glance,openstack/glance,dims/glance,rajalokan/glance,klmitch/glance,stevelle/glance,openstack/glance,klmitch/glance,openstack/glance,dims/glance