schema: domain: Allow interleave of 'smartcard' subelements

schema: domain: Allow interleave of 'smartcard' subelements

Allow interleave of the top level sub-elements as well as the
subelements in the 'host-certificates' mode. Note that '<interleave>'
doesn't work properly if there's multiple definitions of the same
sub-element in the interleave so for this patch I chose to '<group>' the
'certificate' subelements. Another options would require us to stop
enforcing that there's exactly 3 of them.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>