Rebalance of topics with no config files

[dateutli]

Description

This PR introduces the --generate-config flag for the rebalance command.

The flag would update the main process as outlined below:

• From the inventoried config files we create a set to keep track of the existing config files
• Bootstrap missing topic configs
• Re-inventory config files to take into account the newly generated ones
• Continue the process as usual (if a topic config file already existed it'll take precedence and the generated one will be dismissed)
• Clean up bootstrapped topic configs

This PR also adds a wrapper function around the traversal of topic configs for reusability purposes.

Notes

• This is effectively calling the main functionality of the bootstrap command in a limited capacity. We could potentially add support for all of the bootstrap arguments but that's outside the scope of this feature

Testing

1. Picked topic check-topic--JNKfqh for testing
2. $ topicctl get balance 'check-topic--JNKfqh' --zk-addr=localhost:2181

Broker replicas:

ID	RACK	POSITION 1	POSITION 2	TOTAL
1	zone1	2	1	3
2	zone1	1	2	3
3	zone2	1	2	3
4	zone2	2	1	3
5	zone3	1	2	3
6	zone3	2	1	3

Broker rack replicas:

RACK	POSITION 1	POSITION 2	TOTAL
zone1	3	3	6
zone2	3	3	6
zone3	3	3	6

3. Manually bootstrapped check-topic--JNKfqh in order to remove one broker

4. $ topicctl apply 'examples/local-cluster/topics/check-topic--JNKfqh.yaml' --rebalance --to-remove 1

5. $ topicctl get balance 'check-topic--JNKfqh' --zk-addr=localhost:2181

Broker replicas:

ID	RACK	POSITION 1	POSITION 2	TOTAL
1	zone1	0	0	0
2	zone1	2	2	4
3	zone2	1	2	3
4	zone2	2	2	4
5	zone3	2	2	4
6	zone3	2	1	3

Broker rack replicas:

RACK	POSITION 1	POSITION 2	TOTAL
zone1	2	2	4
zone2	3	4	7
zone3	4	3	7

7. Deleted manually bootstrapped topic config

8. $ topicctl rebalance --cluster-config='examples/local-cluster/cluster.yaml' --path-prefix='examples/local-cluster/topics' --bootstrap-missing-configs

Broker replicas:

ID	RACK	POSITION 1	POSITION 2	TOTAL
1	zone1	2	1	3
2	zone1	1	2	3
3	zone2	1	2	3
4	zone2	1	2	3
5	zone3	2	1	3
6	zone3	2	1	3

Broker rack replicas:

RACK	POSITION 1	POSITION 2	TOTAL
zone1	3	3	6
zone2	2	4	6
zone3	4	2	6

[pazhang-25]

Sorry if you were already planning on doing this, but could you perhaps add a Description section on your PR and also a Testing section to show how you tested the code? :)

[dateutli]

Sorry if you were already planning on doing this, but could you perhaps add a Description section on your PR and also a Testing section to show how you tested the code? :)

My bad, didn't mean to send it out for review just yet.

[petedannemann]

Looks pretty good, left two minor comments

[hhahn-tw]

Please also provide update the README outlining this new feature

[hhahn-tw]

looks good!