partitionedfs: make the interface a bit saner

partitionedfs: make the interface a bit saner

We have 'add_partition()' call to add one partion, but we have 'add_disks()'
method to add all disks at once. This is inconsistent and makes the code more
difficult to understand. Fix this by changind 'add_disks()' into 'add_disk()'
which adds a single disk. This makes things look a lot nicer.

Change-Id: I8a4f4249b1f8fa02486d4cbb3fbc4226ca2e65b8
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>