Create instanceRole per-nodegroup not per-cluster

[lukehoban]

Currently we are creating an instance role per cluster which is reused for each NodeGroup. This is not always sufficient, there are cases where different NodeGroups want different Roles. The default policies will be the same for each of these roles, but users may want to attach additional policies themselves to only some NodeGroups.

This also more closely aligns with the documentation and CloudFormation template used by EKS:

• https://github.com/awslabs/amazon-eks-ami/blob/master/amazon-eks-nodegroup.yaml#L184
• https://docs.aws.amazon.com/eks/latest/userguide/launch-workers.html

[naineel]

We are still blocked on this. What's the timeline on the release of 0.22 ?