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.
You also need to copy `:foreman_url` as well as `:template_url` from my original PR. One is the URL the proxy uses to contact foreman, the other is the URL that should be written into the templates so that the host knows where to find the proxy.