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.
@dustints thanks, but I still feel we want to refactor Renderer as a separate PR. This feels like scope-creep. @eLobato what do you think?