Release for k8s 1.18

[rhockenbury]

What would you like to be added:
An official release for deploying k8s 1.18

Why is this needed:

[johnjeffers]

Seconded, please and thank you!

[floryut]

2.13 with 1.17 will be released soon, work on 1.18 on master will follow, sorry guys for lagging a bit behind lot of work to be done

[floryut]

By the way @rhockenbury and @johnjeffers if you can try to use master with kube_version set to v1.18.1 and report any issues that'd help us ;)

[johnjeffers]

@floryut I have good news. The last time I tried it, the playbook would fail when it tried to retrieve kubeadm. But it seems to be working now! I was able to bring up a 1.18.1 cluster by manually updating kube_version.