hack/terraform-quickstart: default root disk size to 30GB
hack/terraform-quickstart: default root disk size to 30GB

This matches the defaults in tectonic-installer and it means we don't
run out of disk space building conformance tests on a node.

dghubble/bootkube,kubernetes-incubator/bootkube,hongchaodeng/bootkube,dghubble/bootkube,diegs/bootkube,kubernetes-incubator/bootkube,diegs/bootkube,ericchiang/bootkube,kubernetes-incubator/bootkube,ericchiang/bootkube,diegs/bootkube,hongchaodeng/bootkube,ericchiang/bootkube,dghubble/bootkube,hongchaodeng/bootkube