Instance type order preference not respected

[xeivieni]

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Describe the Bug

As stated in the AWS node group documentation here https://docs.aws.amazon.com/eks/latest/userguide/managed-node-groups.html

Managed node groups use the order of instance types passed in the API to determine which instance type to use first when fulfilling On-Demand capacity. For example, you might specify three instance types in the following order: c5.large, c4.large, and c3.large. When your On-Demand Instances are launched, the managed node group fulfills On-Demand capacity by starting with c5.large, then c4.large, and then c3.large

However, the module overrides the order of the instance_types list by doing a sort on it https://github.com/cloudposse/terraform-aws-eks-node-group/blob/master/main.tf#L74

Expected Behavior

The order of preference of the instance_types shall be passed to the node_group resource as specified by the user

Steps to Reproduce

Steps to reproduce the behavior:

1. Create a node group with three instance types
2. Apply
3. Change the order of the instance types
4. plan
5. See no changes

[Nuru]

closed by #142