restapi: Fault detail is optional

The "detail" field of faults is optional, but in the XML schema it isn't
defined as such because the "minOccurs" attribute isn't specified and
the default is 1, which means mandatory.

Change-Id: I3145e3e2d6a242869cfd0edee5fa70a07c6eb649
Bug-Url: https://bugzilla.redhat.com/1102050
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>