Use managed instance groups for server and client deployments

[picatz]

It would probably be a good idea to use managed instance groups for Nomad server and client deployments in the future with the compute_instance_group_manager Terraform resource. GoogleCloudPlatform/terraform-google-managed-instance-group looks like a good starting point.

Need to look into how that works, implement it, and then document how to trigger rolling updates for Nomad.