Apply recommended system settings

[luissimas]

Description

Incus's docs mention some recommended system settings that should be set to avoid hitting Linux resource limits. The charm should be able to apply those configurations on the host. We should also expose a config option to disable the automatic application of those system settings.

The settings mentioned in the documentation are composed of sysctl parameters and /etc/security/limits.conf. For the sysctl part, it is a good idea to check if the sysconfig charm already does what we need and, if it does, we shouldn't replicate that in the charm.

We should also consider which settings require a system restart to take effect, and probably enter a blocked state and then prompt the user for a manual restart. Automatic restarts should be avoided especially on a hypervisor host, which might be used by other Juju units such as Ceph OSDs and OVN chassis that might have their initialization process disrupted by an automatic restart.

Definition of done

• A config option apply-system-settings or similar is present in the charm with the default value true
• If the option is set, the charm applies the recommended system settings on the host

Resources

https://linuxcontainers.org/incus/docs/main/reference/server_settings/
https://charmhub.io/operator-libs-linux/libraries/sysctl
https://charmhub.io/sysconfig