restapi: LUN disk creation - storage is not applicable

Storage domain is not applicable for DirectLUN disks,
hence, removed from 'optionalArguments' on rsdl.

Change-Id: Ie1a28b47096016b4a5293c349399c98862978d4a
Bug-Url: https://bugzilla.redhat.com/1083096
Signed-off-by: Daniel Erez <derez@redhat.com>