* Change Google Cloud module to require the `region` variable
* Workers are created in random zones within the given region
* Tolerate Google Cloud zone failures or capacity issues
* If workers are preempted (if enabled), replacement instances can
be drawn from any zone in the region, which should avoid scheduling
issues that were possible before if a single zone aggressively
preempts instances (presumably due to Google Cloud capacity)
* Terraform provider "google" plugin releases leave the disk
device_name as "" by default. Recently the API has started to
set a default name "persistent-disk-0". Plan and apply show
all instance groups need to be recreated to "fix" the name
* Impact: Controller and worker instance groups are deleted
and recreated, deleting data on controllers and bringing
down clusters
* Fix: Explicitly set the disk_name to persistent-disk-0 so
that terraform finds no diff needs to be applied.
* https://github.com/poseidon/typhoon/issues/34
* https://github.com/terraform-providers/terraform-provider-google/issues/574
