Add a new mode to the scripted partition editor for variant disk names.

If the disk parameter "DEFAULT" is set in place of an actual device name,
or no disk is specified for the PARTITIONS parameter, the installer will
follow the logic used in the automatic-partitioning mode, in which it
will either provide a selection dialog for one of several disks if
several are present or automatically select it if there is only one. This
simplifies the creation of fully-automatic installation media for
hardware or VMs with varying disk names.

Suggested by: Egoitz Aurrekoetxea <egoitz@sarenet.es>
MFC after: 3 weeks
Relnotes: yes