imdsclient: return a single hostname from fetch_hostname

[zmrow]

Issue number:

Related to #3031

Description of changes:

The fetch_hostname method queries the metadata/local-hostname IMDS target, which has the potential to return multiple space-delimited hostnames. This changes the method to return only the first hostname in the list.

Testing done:
Build and booted an aws-k8s-1.24 node into a VPC setup with a DHCP option set with multiple domain names.

Confirmed IMDS returns multiple hostnames from the meta-data/local-hostname endpoint:

[ec2-user@admin]$ curl -H "X-aws-ec2-metadata-token: $TOKEN" http://169.254.169.254/latest/meta-data/local-hostname
ip-192-168-23-243.us-west-2.compute.internal fdjskl.com abcdefg.com

Confirmed my node has the proper hostname set in /proc/sys/kernel/hostname and in settings.network.hostname

bash-5.1# cat /proc/sys/kernel/hostname 
ip-192-168-23-243.us-west-2.compute.internal
bash-5.1# apiclient get settings.network.hostname
{
  "settings": {
    "network": {
      "hostname": "ip-192-168-23-243.us-west-2.compute.internal"
    }
  }
}

Confirm the node properly joins the cluster and has the expected name:

$ kubectl get nodes
NAME                                           STATUS   ROLES    AGE   VERSION
ip-192-168-23-243.us-west-2.compute.internal   Ready    <none>   18m   v1.24.12-eks-76dc719

Also turned on RBN in my subnet and launched another instance to validate we see the same correct behavior:

[ec2-user@admin]$ curl -H "X-aws-ec2-metadata-token: $TOKEN" http://169.254.169.254/latest/meta-data/local-hostname
i-009d9789c65c9a4a4.us-west-2.compute.internal fdjskl.com abcdefg.com

[ec2-user@admin]$ cat /proc/sys/kernel/hostname 
i-009d9789c65c9a4a4.us-west-2.compute.internal

[ec2-user@admin]$ apiclient get settings.network.hostname
{
  "settings": {
    "network": {
      "hostname": "i-009d9789c65c9a4a4.us-west-2.compute.internal"
    }
  }
}

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[stmcginnis]

Assuming testing goes well, this looks good to me.

I don't actually think it would be worth it, but I noticed there is a library out there that could be used to set up some unit testing for this. Just pointing out in case it is helpful in any way with manually testing things.

[zmrow]

Updated the testing in the description and pulled out of draft

[zmrow]

Added additional testing on a subnet with RBN enabled.

[arnaldo2792]

Typo in the commit, potentiall.

[zmrow]

^ Fixed the typo in the commit message. Thanks @arnaldo2792