Make ec2:ssh use the ssh key if configured

Hi!  First off, thanks so much for this tool!

I typically don't program in Ruby, so I'm not sure if I'm doing everything the best way possible here, but I've made a few additions.  All of these are working here, but not knowing ruby or capistrano well, I haven't tested them in different circumstances, but I think they should be compatible.  If you have any questions / comments / change requests, I'm totally open!

This pull request add support for `Net::SSH`'s [ssh_options[:keys]](http://net-ssh.github.io/ssh/v1/chapter-2.html).