Add support for GCP preemptible instances

[tomwhite]

It would be useful to be able to try some workloads on a cluster with preemptible workers.

Ideally the worker would try to shut down gracefully, but I've successfully some workflows without this (see https://github.com/pystatgen/sgkit/issues/453). Also, it's not clear if the 30s termination warning is enough time to save the worker memory state in practice, although it would still be worth the worker shutting down on a best effort basis.

[tomwhite]

Fixed in #256