[New Feature] Allow choosing EC2 types when deploying an EKS cluster

[LucaCinquini]

Currently, the selection of EC2 types for an EKS cluster is hardcoded into this file:

https://github.com/unity-sds/unity-sps/blob/develop/terraform-unity/modules/terraform-eks-cluster/main.tf#L6-L11

We want to make all those 4 fields configurable parameters in the tfvars file, with sensible defaults (probably the current values).

[LucaCinquini]

Drew has updated the TF scripts to choose the type and size of the EC2 worker node group. Luca tested deploying a new EKS cluster with the new configuration.