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

An update to @GregSutcliffe's original PR (https://github.com/theforeman/smart-proxy/pull/100/files). Ports his original feature to the new plugin api.
Given the oddball situation that would occur if this were a plugin (code merged to Foreman core depending on a separately packaged proxy plugin), and given the Foreman rendered code would need rewriting for the core side code to be a plugin, I'm changing my vote to align with Ohad - this should be merged in to the smart-proxy rather than packaged separately. I think it's sufficiently useful to users to segregate networks to have it merged in (but disabled) by default.