paritionedfs: improve readability

paritionedfs: improve readability

Rename the 'disk' key in parition and subvolume dictionaries into 'disk_name',
because it is really a disk name string. Old name is very confusing because
the disks dictionary also has a 'disk' key which points to a real disk object.
I was many times confused by this when I was trying to understand the code, and
now it is time to clean the mess up a bit.

Change-Id: I58526ce719f831051924498550df5de9b3ebd737
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>