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.
@GregSutcliffe i think i can get around this by renaming `forman_url(action)` to `_foreman_url(action)` in ForemanUrlRenderable
and then define foreman_url(action='built')/foreman_url(action='provision') that call `_foreman_url(action)` in `Foreman::Renderer` and `HostTemplateHelpers` respetively