rest: fix xsd validation for Payload

volume id is optional parameter for payload,
so setting minOccurs to 0

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1031960
Change-Id: Ib74b032753a11f972ea82b89a69044d68b3c76ed
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>