ceph-volume: prepare/create - size args as Size class

ceph-volume: prepare/create - size args as Size class

This add the disk.Size class as all size related argument types. We
often create this class form args like this anyway and it enables users
to pass not only bytes but also strings like 50G.

Signed-off-by: Jan Fajerski <jfajerski@suse.com>
(cherry picked from commit 2ff37321640a614fd965d7df6e20d8f9d1430fa3)