allow setting label to nodes about to be upgraded/restarted

Description

Because there is no agreed-upon way to signal operators that a node is drained, there are multiple ways that operators handle it.
Rook detects node drain by observing pods on the node. This works fine but feels a bit fragile.
The problem is that some operators (e.g. the Zalando PostgreSQL Operator) "detect" drains by watching node's labels. Whenever a label is not set anymore (e.g. "node-ready=true") it will (try to) failover to another DB pod on another node.

This is a feature request to update node's labels when a reboot is about to happen.

Steps to reproduce the issue:

1. update some machineconfig,
2. observe machine-config-daemon trying to drain a node,
3. failing to drain the node because there is a pdb on a pod on that node,

meanwhile
4. some operator not knowing that the machine is about to be rebooted and not updating the pdb (directly or indirectly.)

5. the node not getting drained.

Describe the results you expected:

1. update some machineconfig,
2. machine-config-daemon updating label machineconfiguration.openshift.io/pending-restart=false to =true,
   3a. an operator removes active workload from the node, removing/updating pdbs that affect the node,
   3b. machine-config-daemon drains the node,
3. node reboots successful,
4. machine-config-daemon sets label machineconfiguration.openshift.io/pending-restart=false.

Start with the machine-config-daemon workflow described in the issue, especially the node drain and reboot sequence triggered by a MachineConfig update. Trace where drain attempts begin and where reboot completion is recorded; done means the pending-restart label changes before draining and returns to false after a successful reboot, with behavior verified around PDB-blocked drains.