Autoscaling: Move target count independent of actual count.

This should let us grow and shrink the worker pool in chunks when necessary
(like when a bunch of them go offline, or when there is a worker version
change).
Fix import
NoopTask: Fix things that should be static. Add simple factory method.
Autoscaling changes from code review.

- Log and return immediately when workerSetupData is null
- Allow provisioning more nodes while other nodes are still provisioning
- Add tests for bumping up the minimum version
Autoscaling: Terminate obsolete workers faster
Merge pull request #339 from metamx/autoscaling

Autoscaling: Move target count independent of actual count.