Login to the corresponding (self.url) mattermost instance.

def login(self, login, password):
        """Login to the corresponding (self.url) mattermost instance."""
        props = {'login_id': login, 'password': password}
        p = requests.post(self.url + '/users/login', data=json.dumps(props))
        self.token = p.headers["Token"] # Store the token for further requests
        self.get_team_id()
        return json.loads(p.text)