User management

[errordeveloper]

At the moment we create clusters dedicated for single user, we need to extend this.

https://docs.aws.amazon.com/eks/latest/userguide/managing-auth.html

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

It'd be nice to have two features:

• add any existing IAM user to a cluster
• add all admin (as in, those with Administrator policy) to a cluster when it's created (may need sync loop)

[christopherhein]

Couple things to keep in mind as we move forward the potential of using CRDs for user management in the authenticator - kubernetes-sigs/aws-iam-authenticator#79

[errordeveloper]

xref #544