Refs #969 - Foreman-side changes for serving templates from the proxy

Needs the proxy-side PR to function.

This is a rebase of @GregSutcliffe's original https://github.com/theforeman/foreman/pull/751.
I agree, but it's going to involve a refactor of unattended_controller to understand a new parameter, which is slightly more extensive. This works as is, has been tested, and the extra request to the proxy for template_url is fairly lightweight. I'd also like to include Dustin's refactor of the renderer too, so we can definitely improve this stuff in the near future.

If you're agreeable, we can get this in, and then see how much work the changes to unattended_controller are? I'll raise a redmine for that if so.