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.
Actually, in my testing uri.host is still getting unset because we're only passing the path. why are we not using `Net::HTTP::Get.new(uri)` ?