Add support for Kubernetes node draining

[dghubble]

• Zincati requests include a node (i.e. agent) UUID identifier, assigned matching systemd-id128's app-specific UUID behavior. Replicate the same logic used in systemd and Zincati (rust) to allow systemd machine IDs (available via Kubernetes Nodes
  System UUID info) to be mapped to the ID Zincati would assign. This allows a specific Zincati request to be mapped to the
  matching Kubernetes node

Behavior

• When a node prepares for reboot, Zincati sends a lock request, and if a lock is obtained (or still held, since protocol allows
  reentrant locks) cordon the Kubernetes node and drain it by evicting Pods (except mirror or DaemonSet Pods)
• When a node reaches steady state, Zincati sends an unlock request and if any lock was previously held, it is released
  and the node uncordoned (reversing earlier cordon or possibly a cordon applied by another system)

Caveats

• Drain immediately creates Eviction objects, which are responsible for evicting Pods respecting different constraints
• At the moment, fleetlock does not wait for Pods to be terminated. Similarly, locks are still given out, even if draining fails. For now, I'd rather not gate lock grants on successful drains, since unknown termination edge cases could wedge a cluster's auto-upgrades. This is still an improvement over no draining (previous)

[lucab]

Sorry that I didn't notice this earlier. If you happen to need more flexibility, you can also freely customize the agent ID through the identity.node_uuid config parameter (documented here).

[dghubble]

Thanks @lucab. This drain feature would only work for auto-assigned node_uuid's at the moment, since its predictable and folks don't have to configure anything. But good to know about, maybe that becomes useful in future 👍

[perosb]

Thanks for adding this @dghubble

I'm not sure how to report issues in this repo?
There seem to be some mimatch with the ID matching.
It seem SystemUUID is passed to ZincatiID but it should be MachineID? I tried machineId in a local build and it matches the node and drains.

Also, the nodes_test.go is testing machineID as input which works.