login(username: string, password: string): Observable<boolean> {
  const url = this.getUrl('/v1/users/login');
  const headers = {
    headers: new HttpHeaders({'Content-Type': 'application/json', 'transactionId': uuid.v4()})
  };
  const body = { username, password };

  return new Observable<boolean>( observer => {
    this.http.post<User>(url, body, headers).subscribe(
      (user: User) => {
        console.log('login: Success: ', user);
        observer.next(true);
      },
      (err: HttpErrorResponse) => {
        console.log('login: Error: ', err);
        observer.next(false);
      }
    );
  });
}