Remove ssh tcp check

When the Docker server builds a new container, Kitchen Docker calls the [test_ssh](https://github.com/test-kitchen/test-kitchen/blob/349a728dda458184b167e118213ce8a06e7bd777/lib/kitchen/ssh.rb#L264) method to determine whether or not the container is available. test_ssh makes this determination by attempting a TCP check to the container SSH port.

For networks that put limitations on outgoing TCP ports, this causes Kitchen Docker to not work if you are connecting to a remote Docker server. Such networks may allow the use of an [SSH Proxy](http://backdrift.org/transparent-proxy-with-ssh), in which case everything else works with the Kitchen Docker driver except for the TCP check.

I added an option ```:no_ssh_tcp_check``` which defaults to ```false``` and, if set to true, calls the already defined ```container_exists?(state)``` method to determine if the container is available, instead of the TCP check.

I have been testing this for about two weeks now and it is working well for me.