Add configuration options for admin access in AWS EKS clusters

[fgimenez]

We should add a configuration option in which we could define an array of admin users' ARN:

"awsAdminUserARNs": [
  "arn:aws:iam::555555555555:user/admin",
  "arn:aws:iam::111122223333:user/ops-user",
  ...
] 

Then modify this template https://github.com/w3f/polkadot-deployer/blob/master/terraform/aws/outputs.tf#L2-L14 to include them under a mapUsers entry. At the end the configmap should look like:

apiVersion: v1
data:
  mapRoles: |
    - rolearn: arn:aws:iam::555555555555:role/devel-worker-nodes-NodeInstanceRole-74RF4UBDUKL6
      username: system:node:{{EC2PrivateDNSName}}
      groups:
        - system:bootstrappers
        - system:nodes
  mapUsers: |
    - userarn: arn:aws:iam::555555555555:user/admin
      username: admin
      groups:
        - system:masters
    - userarn: arn:aws:iam::111122223333:user/ops-user
      username: ops-user
      groups:
        - system:masters
  ...

Details here https://docs.aws.amazon.com/eks/latest/userguide/add-user-role.html