Broken in 20180429 Snapshot (suspected to be ok before then)

This seems to be a regression of a similar bug seen in the past. Kubic's master
(rightly) does not generate an autoyast.xml with any subvolumes defined,
expecting AutoYAST to use the product defaults.

Since 20180429 AutoYAST no longer seems to be doing this, meaning it's
impossible to bootrap a Kubic cluster because AutoYAST does not setup any of
the btrfs subvolumes

This also means I cannot provide y2logs, as all nodes impacted by this are
impossible to connect to the network (/var is not a subvolume, therefore
cloud-init fails)

Marking this as Critical after discussion with Thorsten Kukuk - this failure is
preventing me from demoing Kubic at KubeCon, so a suggested workaround would be
nice.