feat: associate additional security groups on node creation

[mumoshu]

Adds the --node-security-groups flag, along with the corresponding config key securityGroups, that accepts ARNs of the additional security groups attached to the nodes.

The additional security groups can be used for allowing extra egress from/ingress to pods running on the nodes.

Resolves #397

[mumoshu]

I've created a brand new cluster with eksctl built from the branch running eksctl create cluster --node-security-groups SG_ID. All the EC2 instances in the node group has the security group attached, and the stack template includes the specified security group ID in it:

{"AssociatePublicIpAddress":true,"IamInstanceProfile":{"Ref":"NodeInstanceProfile"},"ImageId":"ami-094fa4044a2a3cf52","InstanceType":"m5.large","SecurityGroups":[{"Ref":"SG"},"sg-0f4e477e7e496aa26"], ...

[errordeveloper]

This is great, thanks for adding it! However, I think this is an advanced use-case and I'd rather avoid adding any new flags beyond what we have already and only enable this via config file. But at the moment, there is no --config-file in eksctl create nodegroup, and we need to work on the API and define standalone NodeGroup object, so not very trivial.

[whereisaaron]

I was looking for this option, and it is already done, which is great, but it is not currently documented in the README.md