Made small improvement for Ubuntu partitioning.

Now user can locate OS partition on any of available
disks on node. But there is still the limitation that
OS volume group MUST be located on one disk. It is
because we do not use LVM for OS partitioning on Ubuntu.

Change-Id: Ieb4f115917722f39ca262c94ed1168d88e7b7a79
Closes-bug: #1261220
Merge "Made small improvement for Ubuntu partitioning."