Force flag not optional for detach storage domain

Default value of force flag for detach sd not set but it should be
optional according to schema.


Bug-Url: https://bugzilla.redhat.com/1134290
Change-Id: Ia0b7b34464ec83bbf408df61c0193cd17e193e00
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/32212
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>