Inventory-based SSH proxy/bastion host support, ssh_args + docs

Supersedes #9122 

First-class inventory var support for proxy/bastion/jump hosts seems to be increasingly requested. I've tried to make it reasonably comprehensive and well-documented, though I intentionally omitted proxy_password support (had some trouble stacking the file descriptors for the two-level-deep sshpass).

(CC @sivel)

ssh -W asumes openssh-5.4 or later. Older versions of openssh need netcat to achieve this.