Islamic State says it killed tourists in Tunisia

* Army to deploy to major cities after attack * Twenty foreign tourists among dead * Gunmen identified, nine others arrested (Updates with White House statement) By Tarek Amara TUNIS, March 19 (Reuters) - Tunisia said it would deploy the army to major cities and arrested nine people on Thursday after 20 foreign tourists were shot dead in an attack on a museum which Islamic State militants called "the first drop of the rain". Officials did not confirm the militants' claim of responsibility, but s