Added ability to ssh tunnel through proxy

* Added support for .opencafe/ssh.config with ssh.proxy option to specify
     global proxy address
* Can set/override global proxy by setting env var: CAFE_ssh_proxy
* If global proxy is specified, all SSH sessions will tunnel through proxy
* Fixed a few typos in the comments
* Updated based on review comments (using ssh.config instead of engine.config)

Change-Id: I993ed2cb03d0b87794005c2c309353505476c46b