title: Airflow SSHHook without hostkey
body: <p>I'm running Airflow 1.10 and attempting to access an SFTP using the SFTP operator and sensor. This relies on the SSHHook and thus I've created an SSH connection with host, login, password, port, and extra {"no_host_key_check": "true"}. However, I'm still getting the error "paramiko.ssh_exception.SSHException: No hostkey for host &lt;> found."</p>
  
  <p>Has anyone successfully used the SFTPSensor without specifying a hostkey?</p>