Document the CLUSTER_NAME env variable

[hgokavarapuz]

I don't see any documentation about the CLUSTER_NAME env variable but I see that in the code which can be useful.

amazon-vpc-cni-k8s/pkg/awsutils/awsutils.go

Line 669 in 3fba7b6

// If the CLUSTER_NAME env var is present,

[markchalloner]

For others, you can use:

kubectl set env daemonset/aws-node -n kube-system "CLUSTER_NAME=..."

to set environmental variables.

[mogren]

Fixed in #726