Collaboratively and incrementally introduce node-purpose resource label

[consideRatio]

This is a followup to #3397 where our node pools got a node-purpose = core|notebook|dask-worker label added to them unconditionally. The unconditional change will force our node pools to upgrade, which often leads to disruption. Due to this, the change hasn't yet been applied to all our clusters node pools.

Here is how I think we should make a hard-to-do-all-at-once infra change collaboratively over time:

• at one point in time: introduce a temporarily opt-out change and opting out of the change systematically
• incrementally over time and by all engineers: incrementally stop opting out of the change across our clusters
• cleanup the temp opt-out config when its not longer referenced

Action points

• Introduce temp opt-out config, apply where needed along with a fixme note
  terraform, gcp: add opt-out var to collaboratively and opportunistically apply node-purpose infra labels #3406
• Collaboratively and opportunistically stop opting out
• Cleanup temp opt-out config

Clusters with partial upgrades remaining currently

• cloudbank
• leap
• pangeo-hubs
• pilot-hubs
• qcl