kubekins-e2e dockerfile: fix bash weirdness by using if instead of &&
kubekins-e2e dockerfile: fix bash weirdness by using if instead of &&

&& style condition makes the docker build exit 1 when the variable evaluated is an empty string, this was verified locally with docker build. The fix is using explicit if statement. See where it failed https://prow.k8s.io/view/gs/kubernetes-jenkins/logs/post-test-infra-push-kubekins-e2e/1364342488625582080

jessfraz/test-infra,kubernetes/test-infra,dims/test-infra,cblecker/test-infra,cjwagner/test-infra,jessfraz/test-infra,kubernetes/test-infra,kubernetes/test-infra,BenTheElder/test-infra,monopole/test-infra,BenTheElder/test-infra,dims/test-infra,cblecker/test-infra,fejta/test-infra,cblecker/test-infra,dims/test-infra,fejta/test-infra,dims/test-infra,BenTheElder/test-infra,cjwagner/test-infra,fejta/test-infra,cblecker/test-infra,monopole/test-infra,monopole/test-infra,pwittrock/test-infra,cblecker/test-infra,michelle192837/test-infra,cblecker/test-infra,kubernetes/test-infra,fejta/test-infra,michelle192837/test-infra,jessfraz/test-infra,michelle192837/test-infra,jessfraz/test-infra,michelle192837/test-infra,BenTheElder/test-infra,pwittrock/test-infra,dims/test-infra,fejta/test-infra,cjwagner/test-infra,pwittrock/test-infra,jessfraz/test-infra,pwittrock/test-infra,kubernetes/test-infra,jessfraz/test-infra,monopole/test-infra,cjwagner/test-infra,BenTheElder/test-infra,cjwagner/test-infra,cjwagner/test-infra,BenTheElder/test-infra,fejta/test-infra,pwittrock/test-infra,kubernetes/test-infra,monopole/test-infra,monopole/test-infra,michelle192837/test-infra,michelle192837/test-infra,dims/test-infra