Add Docker 18.06 to the CI tests
Add Docker 18.06 to the CI tests

1883.1.0 from https://coreos.com/releases/#beta
Release Date: September 11, 2018
kernel: 4.14.69
rkt: 1.30.0
docker: 18.06.1
etcd: 3.3.9
systemd: 238
Ignition: 0.28.0

Confirmed that the image is available using:
gcloud compute images list | grep coreos

Change-Id: If1a694ad67ed3daa105093e52c1515053c78a532

kubernetes/test-infra,ixdy/kubernetes-test-infra,cblecker/test-infra,michelle192837/test-infra,shyamjvs/test-infra,monopole/test-infra,jessfraz/test-infra,ixdy/kubernetes-test-infra,monopole/test-infra,cblecker/test-infra,dims/test-infra,cblecker/test-infra,jlowdermilk/test-infra,jlowdermilk/test-infra,michelle192837/test-infra,BenTheElder/test-infra,kargakis/test-infra,shyamjvs/test-infra,cjwagner/test-infra,shyamjvs/test-infra,dims/test-infra,kargakis/test-infra,brahmaroutu/test-infra,ixdy/kubernetes-test-infra,jessfraz/test-infra,cjwagner/test-infra,kubernetes/test-infra,kargakis/test-infra,brahmaroutu/test-infra,krzyzacy/test-infra,shyamjvs/test-infra,michelle192837/test-infra,cblecker/test-infra,cblecker/test-infra,pwittrock/test-infra,lavalamp/test-infra,monopole/test-infra,monopole/test-infra,kubernetes/test-infra,dims/test-infra,fejta/test-infra,fejta/test-infra,michelle192837/test-infra,kubernetes/test-infra,jessfraz/test-infra,pwittrock/test-infra,monopole/test-infra,BenTheElder/test-infra,fejta/test-infra,jessfraz/test-infra,pwittrock/test-infra,kubernetes/test-infra,cjwagner/test-infra,jessfraz/test-infra,kargakis/test-infra,BenTheElder/test-infra,fejta/test-infra,michelle192837/test-infra,jlowdermilk/test-infra,pwittrock/test-infra,cjwagner/test-infra,dims/test-infra,cjwagner/test-infra,fejta/test-infra,monopole/test-infra,kargakis/test-infra,BenTheElder/test-infra,ixdy/kubernetes-test-infra,jlowdermilk/test-infra,krzyzacy/test-infra,dims/test-infra,michelle192837/test-infra,jlowdermilk/test-infra,pwittrock/test-infra,lavalamp/test-infra,shyamjvs/test-infra,krzyzacy/test-infra,cblecker/test-infra,dims/test-infra,lavalamp/test-infra,brahmaroutu/test-infra,BenTheElder/test-infra,ixdy/kubernetes-test-infra,BenTheElder/test-infra,brahmaroutu/test-infra,krzyzacy/test-infra,jlowdermilk/test-infra,kargakis/test-infra,krzyzacy/test-infra,lavalamp/test-infra,krzyzacy/test-infra,jessfraz/test-infra,lavalamp/test-infra,fejta/test-infra,lavalamp/test-infra,brahmaroutu/test-infra,shyamjvs/test-infra,cjwagner/test-infra,brahmaroutu/test-infra,kubernetes/test-infra