[EKS] [request]: Managed Node Group and Launch template with instance type

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**Tell us about your request**
Allow to have launch template with instance type used in node group with list of instance types.

**Which service(s) is this request for?**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Was trying to have a node group having good selection of burstable instance types without being hit by CPU credits limit.

Usually when using launch template with ASG you can put an instance type in template and then put list of instance types in the ASG it self which uses the template. Since for some reason it is not allowed to both put a single instance in launch template and list of instance types in managed node group which uses this launch template, it is not possible to have T2 instances with unlimited CPU credits. The CPU credits definition seems to be discarded when not having any T2/T3 instance type in template.

**Are you currently working around this issue?**
Using only T3 which has unlimited credits by default.

Start with issue #1352 and its description of EKS managed node groups, launch templates, Auto Scaling instance-type lists, and T2/T3 CPU-credit behavior. No repository files or tests are identified in the issue; done would mean defining and implementing support for the requested instance-type combination, with validation of the resulting node-group behavior.

Written by the indexing model from the issue text.