TPU provisioner should be configurable to stop new nodepool create

[kyle-google]

In the use case of configuring TPU provisioners with multiple clusters and/or potentially version breaking changes, it would be helpful to have a configuration setting to allow the TPU provisioner to run and cleanup, but not get triggered to create anymore nodepools.

Examples:

• Migrating from pod trigger to jobset trigger for nodepool creations and running multiple iterations.
• TPU provisioner needs to run on a cluster to cleanup.
• Temporarily stop the provisioner from creating nodepools, while cleaning up existing nodepools to create capacity.