Include build_index in ClusterConfig json dump

[fgalbnv]

Issue
When the clusters.json file is dumped, it is missing the value for build_index. This is due to the save() function not dumping the value within cluster_config.py:

    def save(self):
        return {'hostname': self.hostname,
                'port': self.port,
                'name': self.name,
                'version': self.version,
                'certificate': self.certificate,
                'private_key': self.private_key}

User impact
If a user encounters an error during the execution of bright_cluster, the user might need to delete the clusters.json file. Otherwise, as the value of build_index is missing from the file, it will be left to None, and cause misbehaviors in the script such as a wrong value returned by cluster_pythoncm_directory().