**OLD** [DPE-4575] Add voting settle logic at start and stop service

[phvalguima]

Currently, we are having several issues with 2-node clusters as OpenSearch won't automatically manage voting anymore. This PR will add logic to manually manage the 2-node cluster scenario using the voting_exclusions API. Whenever we have only two nodes active and registered to the cluster as voting units, _settle_voting_exclusions will exclude the non-leader unit from voting. It also excludes the leaving unit, as we need to cover for the scenario where that unit is the cluster_manager and moving from 3->2 units we may end up with stale metadata.

This PR also makes exclusion mandatory to happen at start / stop. Therefore, we are sure the voting count will always be correct in each stage.

For example, moving from 3->2->1 units results:
3->2) The cluster will set 2x voting exclusions: one for the unit leaving (if this is the cluster manager, that position will move away) and one for one of the 2x remaining units, following a sorted list
2->1) The voting exclusions are removed

Likewise, on scaling up:
1->2) One voting exclusion is added, following the same sorted list of all node names
2->3) Voting exclusions are removed

This PR touches #324, #326, #327 and in #325 this behavior is also observed. This is also linked to issues in our ha/test_storage.py, has one can see in this run.

Closes: #324, #326, #327

[reneradoi]

Hey @phvalguima I've tested this locally and ran into issues when removing the application with only two units left. It runs into

unit-opensearch-3: 09:41:16 ERROR unit.opensearch/3.juju-log Uncaught exception while in charm code:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-opensearch-3/charm/./src/charm.py", line 267, in <module>
    main(OpenSearchOperatorCharm)
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/main.py", line 544, in main
    manager.run()
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/main.py", line 520, in run
    self._emit()
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/main.py", line 509, in _emit
    _emit_charm_event(self.charm, self.dispatcher.event_name)
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/main.py", line 143, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/framework.py", line 350, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/framework.py", line 849, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-opensearch-3/charm/venv/ops/framework.py", line 939, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-opensearch-3/charm/lib/charms/opensearch/v0/opensearch_base_charm.py", line 523, in _on_opensearch_data_storage_detaching
    raise Exception("Unable to acquire lock: Another unit is starting or stopping.")
Exception: Unable to acquire lock: Another unit is starting or stopping.

and can't get out of it anymore. Furthermore I've seen some failures on the integration tests, so it looks like it does not really work yet.

[reneradoi]

Looks good!