ceph-disk: implement list --format json

ceph-disk: implement list --format json

The ceph-disk list command is reworked in two parts:

 1) the list_devices function that build an internal structure with all
    the information regarding disks and partitions.
 2) a function to display the internal structure in plain text or json

The ceph-disk list show the plain text version and is backward
compatible.

The ceph-disk list --format json output has more information about each
device than the plain text version and is intended for scripts.

The unit tests cover all modified lines (2610 to 2849).

Signed-off-by: Loic Dachary <ldachary@redhat.com>