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.
Forgot to mention, this is very similar to what we do in the TFTP api, where we have a /bootServer endpoint, which the *DHCP* orchestration in Foreman uses to write next-ip data in the host reservations.