Add a user to config.

def add_user(
        self,
        name, 
        **attrs
        ):
        """Add a user to config."""
        if self.user_exists(name):
            raise KubeConfError("user with the given name already exists.")

        users = self.get_users()
        
        # Add parameters.
        new_user = {'name': name, 'user':{}}
        attrs_ = new_user['user']
        attrs_.update(attrs)
        users.append(new_user)