aws: limit the scope of IAM instance policy

[vaijab]

Current AWS implementation allows for compute and master nodes to manage pretty much all EC2 resources across AWS account, see:
https://github.com/UKHomeOffice/keto/blob/master/pkg/cloudprovider/providers/aws/cloudformation_templates.go#L356
https://github.com/UKHomeOffice/keto/blob/master/pkg/cloudprovider/providers/aws/cloudformation_templates.go#L533

compute nodes

• should only be able to describe their own tags. I cannot think of anything else that they need?

master nodes

• whatever kubernetes requires
• should be able to set tags on compute nodes as part of the same cluster

[jon-shanks]

Makes sense to me , we definitely should limit the compute nodes. Would be good to limit the master but less of a security risk to a degree but probably worth investing the effort in both