Adds ssh gateway option to ec2 provider closes saltstack/salt#9782

This adds the ability to specify a ssh gateway host so that your salt-master does not have to be on the same private network as your minion.

I've only added the functionality for the ec2 provider as I don't have access to the other clouds to develop with, however, each provider can take advantage of the ssh gateway by adding the options to the provider. No work should be need elsewhere.