[doc] Describe how to extract an fs UUID from an image
[doc] Describe how to extract an fs UUID from an image

For software RAID, the image needs to have the root filesystem UUID
as metadata. This patch describe how to extract this UUID from an
existing image.

Change-Id: Ifdb63a80d4138a41fd0bac9880d14c90a4378fe5

openstack/ironic,openstack/ironic