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.
I don't think we formally agreed on this, but I think (and I tried to follow this logic in other parts of smart-proxy) it's better to fail-fast: if a required piece of configuration is missing, the module shouldn't be loaded at all, and an appropriate message should be logged.