consider using etcdv3 terraform backend instead of s3

[kevindrosendahl]

https://www.terraform.io/docs/backends/types/etcdv3.html

pros:

• consistent across cloud providers
• don't have to provision an s3 bucket, etc, already need etcd
• built in state locking

cons:

• may be more "expensive" ($/bit, will add more load (although probably not much) to etcd)
• not sure what the versioned history story is
• probably not portable across hosted k8s (they likely don't give direct etcd access)

if we stick with s3, consider enabling locking with a dynamodb table: https://www.terraform.io/docs/backends/types/s3.html