Add support for multi-pools for workers

[aarontp]

We can add different worker pools that are created to match individual job types (e.g. so Plaso can have worker nodes that have more cpu than workers for other job types). Right now it's one global worker pool.

[wajihyassine]

From a GKE autoscaling perspective, we can consider:

• 1 Large Task gets a single node for processing
• Multiple small tasks can run multiple pods on a single node

Also handy resource: https://medium.com/google-cloud/scale-your-kubernetes-cluster-to-almost-zero-with-gke-autoscaler-9c78051cbf40

[jleaniz]

Tagging 'celery' because I think we can use built-in functionality for this