Use eni-max-pods mapping file for setting of the max pods value

**What I'd like:**
It is possible to set the static max_pods to a customised value. But it doesn't cater for node groups with instance types that need different settings for the max_pods value. It should be possible to set max_pods dynamically based on the values in the eni-max-pods mapping file.

There should be a configuration so that the max_pods value for an instance gets derived from https://github.com/awslabs/amazon-eks-ami/blob/master/files/eni-max-pods.txt according to the instance type.