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.
@dustints, @GregSutcliffe -- an example of disabling a plugin on startup: https://github.com/theforeman/smart-proxy/blob/develop/modules/facts/facts_plugin.rb#L8