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.
It looks like Addressable::URI in combination with Net::HTTP.new would allow a bare IP or hostname, or bare IP or hostname : port, in additional to a fully specified URI. I might suggest preventing non-fully-qualified URIs?

Suggested replacement commnet, because "templates are always retrieved on http" is a false statement (you can retrieve by https).
``` ruby
# The default protocol is http on port 80 unless otherwise specified in the url.
# Examples:
# foreman.example.com:3000
```