[DPE-2198] Integrate sysctl lib

[zmraul]

Integrate sysctl lib into Kafka charm.

Proposed "default" values for os config are:

OS_REQUIREMENTS = {
    "vm.max_map_count": "262144",
    "vm.swappiness": "1",
    "vm.dirty_ratio": "80",
    "vm.dirty_background_ratio": "5",
}

[rgildein]

I think this is good, there is only one issue with charm config, which should be removed if anything failed.

[deusebio]

First pass.

I have two major points:

1. I would try to re-write slightly the business logic for update to make validation in memory and keep the status as clean as possible. I have a proposal in the comments that I believe would make things more robust.
2. I believe it would be best to encode in the library the removal of charm_filepath (and possibly creation, even with just empty content, just to make sure it is written in the header of the merged sysctl file). For doing this we should add some handlers to _on_install and _on_remove events (such that this is done by default and charm authors do not forget)

[deusebio]

LGTM!

[rgildein]

Looks good to me, thanks for cooperation.