extract method: load_host_config
use UR::Util::load_class_or_file

Use UR::Util::load_class_or_file is meant to allow you to try to use a
module and to pass through if the file is missing but not if it can't
compile, etc.
move module_to_filename to test

Move module_to_filename to the test, where it is used, since it is no
longer used in the module.