prometheus::daemon::real_download_url should allow empty string

[TheMeier]

prometheus::daemon::real_download_url is not used when install_method is 'package'
it should be optional in prometheus::daemon and the prometheus::*_exporter classes

[bramblek1]

I would argue this type safety should be left to the archive resource that gets the file.

When using prometheus::daemon to add non-standard exporters which are already installed, I have used a workaround.

install_method: "none"
real_download_url: "http://nowhere/"

It would also be great to be able to provide a file:// url which would be useful for environments where network access is restrictive.

edit - change provider to install_method in example; spelling