Support spot instances in worker module

[rfrenchseti]

• Investigate how to handle spot instances that have "terminated". Can we re-start them?
• GCP gives 30 second warning for spot termination.
  • Log it.
  • Set a flag for the apps.
  • They should check the flag before committing final results so it's atomic.
• Do we automatically create new instances as appropriate?

[rfrenchseti]

Logging and flag are done.