CVE-2018-20321: Exposure of Resource to Wrong Sphere

An issue was discovered in Rancher 2 through 2.1.5. Any project member with access to the default namespace can mount the netes-default service account in a pod, and then use that pod to execute administrative privileged commands against the k8s cluster. This could be mitigated by isolating the default namespace in a separate project, where only cluster admins can be given permissions to access. As of 2018-12-20, this bug affected ALL clusters created or imported by Rancher.