fixes #1937 - Foreman doesn't work well when run on subpath

• request_url is not enough to pass form controller to tftp
  Other variables (like script_name) need to be used from
  controller.url_options to properly generate the path in the PXE template.
• use rails helper to get the path to noVNC source
  Hardcoded value makes problems when running Foreman from subpath
• helper javascript method for generating path to foreman