Asthma can be divided into three categories:mild,moderate and severe.If u have severe asthma,then it can be life threatening then there is no use of hiding it.its better to tell them honestly.Because health and ur precious life comes first.Asthma doesnt lead to disqualification.But if u have severe asthma,telling them can be helpful in ur future life,like place of posting can be associated with ur problem.By theway,u should opt for foreign service as ur first preference.That would be perfectly in line with ur health and suitable lifestyle.And change of places has proven to remove allergies in many cases.