Just a thought on the delimited `String` versus `List<String>` choice - couldn't we add a type coercion that splits the string appropriately? Not sure how we'd handle different delimiters, but settling on `,` is probably OK for now. Just adding this to [TypeCoercions.java](https://github.com/brooklyncentral/brooklyn/blob/master/core/src/main/java/brooklyn/util/flags/TypeCoercions.java) ought to work:

    registerAdapter(String.class, List.class, new Function<String,List>() {
        @Override
        public List<String> apply(final String input) {
            return ImmutableList.copyOf(Splitter.on(",").split(input));
        }
    });