This is the backend parts of
!19045.
It adds the following controller actions:

- ClustersController#revoke_aws_role to allow a user
  to modify the role that will be assumed to create
  their EKS resources.
- ClustersController#aws_proxy to populate the cluster
  form with roles, subnets etc from the user's account.
- ClustersController#create_aws to create the cluster
  record and trigger the provisioning workers.