Implement image cleanup and separate it from bot teardown

[canercidam]

• No more removing images in the teardown method
• Implemented separate image cleanup:
  • The supervisor runs the cleanup every minute but the cleanup really executes only every hour.
  • The cleanup accepts an exclusion list (helps us exclude heartbeat).
  • If any image is not a Disco image, it is skipped.
  • If any image is in use by any container (whether it's up or down), it is skipped.