Extends JcloudsLocation customisation hooks

Adds:
* A method with signature `customize(JcloudsLocation, ComputeService, Template)` to JcloudsLocationCustomizer, for use in preference to `customize(.., TemplateOptions)` when the subclass wants to access the template's location as well as its options.
* A `JCLOUDS_LOCATION_CUSTOMIZERS_SUPPLIER_TYPE` config key, that should be the String name of a class implementing `Supplier<Collection<JcloudsLocationCustomizer>>` for use when simple instantiation of a class named in `JCLOUDS_LOCATION_CUSTOMIZER_TYPE` is insufficient.