Open hangs without "Welcome" message

I'm seeing an infinite hang in `_validate_ssh_process`. Here's what I think is supposed to happen: There are two threads, one each for reading from stdout and stderr. Upon stdout receiving a message to stdout, the validation succeeds and the program can continue. 

This seems to expect some kind of a "welcome" message from the service. If there is none, then validation loops forever. This is potentially worse than a validation failure, as it hangs the parent program.  

Here's a small patch to allow skipping this validation entirely - it should probably have a command line option added if others find this useful. I'm currently using this to connect to webserver that's only exposed to 127.0.0.1 on a remote machine.