[EKS] [request]: Document EKS default launch template

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**Tell us about your request**

Extend EKS [Launch template support guide](https://docs.aws.amazon.com/eks/latest/userguide/launch-templates.html) to include the default launch template that EKS uses.

**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?**

I am trying to create a managed node group with an extra EBS disk. Based on https://github.com/aws/containers-roadmap/issues/1199#issuecomment-748296234, I created a launch template with only `BlockDeviceMappings` in its launch template data. Here is the launch template data I used:
```
{
"BlockDeviceMappings": [
{
"DeviceName": "/dev/xvda",
"Ebs": {
"DeleteOnTermination": true,
"VolumeSize": 200,
"VolumeType": "gp2"
}
},
{
"DeviceName": "/dev/sdf",
"Ebs": {
"DeleteOnTermination": true,
"VolumeSize": 500,
"VolumeType": "gp2"
}
}
]
}
```
Then I created the node group using `--launch-template`:
```
aws eks create-nodegroup \
--launch-template name=eks-nodegroup-ebs-extra-disk \
--cluster-name ${CLUSTERNAME?} \
--nodegroup-name ebs-extra-disk \
--scaling-config minSize=0,maxSize=1,desiredSize=1 \
--subnets ${SUBNET?} \
--node-role ${EKS_WORKER_NODE_ROLE?} \
--kubernetes-version 1.17 \
--release-version=1.17.12-20210310 \
--instance-types m5.xlarge
```

Then I inspected the generated launch template of the underlying ASG:
```
ASG=$(aws eks describe-nodegroup \
--cluster-name ${CLUSTERNAME?} \
--nodegroup-name ${NODEGROUP?} \
--query nodegroup.resources.autoScalingGroups[] \
--output text)
LT=$(aws autoscaling describe-auto-scaling-groups \
--auto-scaling-group-name ${ASG?} \
--query AutoScalingGroups[].MixedInstancesPolicy.LaunchTemplate.LaunchTemplateSpecification.LaunchTemplateName \
--output text)
aws ec2 describe-launch-template-versions --launch-template-name ${LT}
```

Then I created another node group using **the same options** but without `--launch-template`. Comparing the launch templates of the underlying Auto Scaling Groups I found the following differences:
| Default | Custom |
| --- | --- |
| `"NetworkInterfaces": [{"DeviceIndex": 0, "Groups": ["sg-...."]}]` | `"SecurityGroupIds": ["sg-..."]` |
| | `"TagSpecifications": [{..."Tags":[{"key": "eks:cluster-name"..."key": "eks:nodegroup-name"}`|
| `"MetadataOptions": {"HttpPutResponseHopLimit": 2}` | |

Copying from docs:
> If you don't specify your own launch template to use when creating a managed node group, the Amazon EKS API creates a launch template with default values in your account.

Based on the above I have the following questions:
1. I would expect the only difference to be the `BlockDeviceMappings`. Why do are we seeing more?
1. What are the "default values" that EKS uses when creating launch templates?
1. In the default configuration, EKS sets `HttpPutResponseHopLimit=2` which means that pods can assume the IAM role of the worker node. Shouldn't this be disabled by default?
1. Is it possible to have a custom launch template and have exactly the same outcome with the default one?

**Are you currently working around this issue?**
In my custom launch template I include the `MetadataOptions` that EKS uses along with my `BlockDeviceMapping`.

**Additional context**
I used an 1.17 EKS cluster and awscli 2.2.35.

**Attachments**
None.

Start with the linked EKS Launch template support guide and the AWS CLI commands and launch-template outputs in the issue. Compare the documented behavior with the reported default and custom configurations; done means the guide explains the default values and the relevant differences, including metadata behavior.

Written by the indexing model from the issue text.