[KIP-460] ElectLeaders implementation public API changes and documentation improvement

[emasab]

• ElectionType in confluent_kafka as in Java
• dictionary value is a KafkaError as it's not thrown
• ElectionType should be added to documentation .rst
• docstring changes
• tests without brokers with successful validation that leads to a timeout
• example elects leaders for all partitions in case no partition is specified

[confluent-cla-assistant[bot]]

:tada: All Contributor License Agreements have been signed. Ready to merge.
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