Add aws-k8s-1.18 variant with Kubernetes 1.18

[tjkirch]

Description of changes:

@sanu11 did the addition of the variant, and I helped track down some testing.

For reference, the addition of 1.17 was in #973.
This one was more straightforward; the only changes:

• We no longer need the patch for aws-sdk-go to support IMDSv2, it's in k8s 1.18.
• We learned that the k8s version skew policy isn't what we thought, so I have another commit fixing the supported control plane versions for each variant in variants/README.md.

Testing done:

• @sanu11 and I both confirmed conformance testing passed on x86, and I confirmed on aarch64.
• Pods run OK.
• I compared system logs and found nothing new.
• I compared kubelet logs and found nothing wrong, however note the next paragraph.

CSI changed a bit in Kubernetes 1.18 (I believe related to kubernetes/kubernetes#84814) and it means that you can't use CSI from a 1.18 node against a 1.17 cluster. I was able to make a 1.18 cluster and confirm that CSI does work.

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.

[tjkirch]

Updated the description. I was able to confirm that CSI works with a 1.18 node against a 1.18 cluster, just not against a 1.17 cluster. (Kubernetes doesn't support using a newer kubelet against an older cluster, as mentioned above, so this seems fine.)