Switch non-redundant check to disk template-based

Currently, the warning/notice about non-redundant instances in cluster
verify is based non empty secondaries list (how old is this?); the
proper way to check this nowadays is via DTS_MIRRORED.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>