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.
Ah i see, the ForemanRequest class references :foreman_url in the main Settings file, but that setting hasn't been added in this PR, so it's assumed to be nil and fails. See comments below about fail-fast for missing settings, and please add the example setting to the PR.