def set_role new_role
      value = make_valid_role new_role 
      return false if !value
      store.set_role(value)      
    end
def set_role new_role
      value = make_valid_role new_role 
      return false if !value
      store.set_role(value)      
    end
Set the role of the subject to a new role
 @param  [Symbol] the role to set
 @return [true, false, Error] true if ok, false if not valid, Error on some error