Split disk watermark check into low and high

[r0mant]

The current satellite high watermark for disk check is hardcoded at 80%. This causes issues for some customers during upgrades from older versions (e.g. 5.0) because this check triggers during an upgrade. Moreover, 80% is a pretty arbitrary value as it depends on the total disk capacity so degrading the cluster doesn't really make sense.

I think we should do the following:

• Have "low" and "high" watermarks. Let 80% be a soft limit and 90% a hard limit (unconfigurable for now for simplicity).
• Low watermark triggers a warning probe.
• High watermark triggers a critical probe (like now).
• Make sure there's a possibility to easily override both of these via a manual knob in emergency. For example via /etc/container-environment variable like other Planet's variables.
• Upgrade command should give user a message if there are any active warnings and alert the user to their presence, but allow to override with --force flag.

The first 4 itemts should be backported back to 5.2. The last one - back to 5.5.

[r0mant]

Released in 5.5.48, 7.0.8. Will be released in next 6.1 as well.