def add_credential(credential_type)
      raise ArgumentError, "The new credential type cannot be nil"   unless credential_type
      raise ArgumentError, "The new credential type cannot be empty" if credential_type.empty?

      new_credential = self.service.object_mask(NetworkStorageCredential.default_object_mask).assignNewCredential(credential_type.to_s)
      
      @credentials = nil

      NetworkStorageCredential.new(softlayer_client, new_credential) unless new_credential.empty?
    end
def add_credential(credential_type)
      raise ArgumentError, "The new credential type cannot be nil"   unless credential_type
      raise ArgumentError, "The new credential type cannot be empty" if credential_type.empty?

      new_credential = self.service.object_mask(NetworkStorageCredential.default_object_mask).assignNewCredential(credential_type.to_s)
      
      @credentials = nil

      NetworkStorageCredential.new(softlayer_client, new_credential) unless new_credential.empty?
    end
Add a username/password credential to the network storage instance