restapi: LUN disk creation - format is not required

'format' is not applicable for DirectLUN disks,
hence, removed from parameters validation.

Change-Id: I35078fb62e5506a51e96d4c99223b8dadd29e5a2
Bug-Url: https://bugzilla.redhat.com/1083096
Signed-off-by: Daniel Erez <derez@redhat.com>