Support ssh_gateway like ssh transport

[atheiman]

This gem is awesome! Im kitchen testing lots of nodes in ec2 and it has dropped my initial converge time from 2 min / node to about 30 sec / node!

It doesnt seem it supports ssh_gateway config like the ssh transport does: https://github.com/test-kitchen/test-kitchen/blob/v1.18.0/lib/kitchen/transport/ssh.rb#L360-L378

I think one of the best use cases for this gem is kitchen tests in cloud providers (ssh transport to a vagrant box is already fast), and most of these setups use a bastion host and private ip'd hosts in a private subnet.

Could this be implemented?

[atheiman]

lol this totally works, i just used it.

[coderanger]

The SFTP plugin is built on top of the core SSH implementation so anything that works in core should work there. For rsync, it opens its own connection so all bets are off.

[atheiman]

One thing i want to take advantage of in Net::SFTP is its ability to list files in the remote filesystem, specifically with wildcards. I opened a pr for it #35