Support for bastions without access to shell

[jackivanov]

Hi there,

It would be great if we can support bastions without shell access

 % knife bastion start
Creating a tunnel to Chef server:
  * Bastion host: [email protected]
  *    Chef host: xxx.local
  *   Local port: 6443
  *      Timeout: 600 seconds
Establishing connection to server.com
Authentication succeeded, local ports should be opened...
OK:  Successfully started proxy on port 6443

% channel 1: open failed: unknown channel type: Access is denied. Opening a shell is forbidden for TCP applications.

% knife bastion status
FATAL: Tunnel is not open on port 6443