API methods returning `Iterable` breaks RESTclient

It is impossible to use the official rest-client module on API methods that return `Iterable`, as Jackson does not know which concrete type to implement. The Jackson configuration is hidden behind RESTeasy, so reconfiguring Jackson is non trivial.

This PR changes some API calls from `Iterable` to `List`, which RESTeasy/Jackson is able to deserialize. Further PRs will need to follow for other `Iterable`-returning API methods.