(#5073) Download plugins even if you're filtering on tags

When we eval a resource in transaction.rb it was being skipped when
filtering on tags and downloading the plugins. There's a lot of
complicated conditions for whether to skip a resource, but this is a
condensed version of the path that was causing plugins not to be
downloaded.

skip?
  missing_tags?
    !ignore_tags?
      !host_config

The Puppet::Configurer::Downloader creates separate catalogs and applies them
to get custom facts and plugins, so should be setting host_config to false.
Puppet::Util::Settings also sets host_config to false when you call use on
settings, while normal catalog application defaults to true.

Thanks to Stefan Schulte <stefan.schulte@taunusstein.net> for suggesting
the implementation fix.