Dump a list of artifacts into a yaml string.

def DumpArtifactsToYaml(self, sort_by_os=True):
    """Dump a list of artifacts into a yaml string."""
    artifact_list = self.GetArtifacts()
    if sort_by_os:
      # Sort so its easier to split these if necessary.
      yaml_list = []
      done_set = set()
      for os_name in rdf_artifacts.Artifact.SUPPORTED_OS_LIST:
        done_set = set(a for a in artifact_list if a.supported_os == [os_name])
        # Separate into knowledge_base and non-kb for easier sorting.
        done_set = sorted(done_set, key=lambda x: x.name)
        yaml_list.extend(x.ToYaml() for x in done_set if x.provides)
        yaml_list.extend(x.ToYaml() for x in done_set if not x.provides)
        artifact_list = artifact_list.difference(done_set)
      yaml_list.extend(x.ToYaml() for x in artifact_list)  # The rest.
    else:
      yaml_list = [x.ToYaml() for x in artifact_list]

    return "---\n\n".join(yaml_list)