FEATURE: Support privileged containers in a pod

Add a Privileged field to containers in a pod, in order to facilitate pods
performing administrative tasks such as builds via Docker-in-Docker.

Discussion: https://github.com/GoogleCloudPlatform/kubernetes/issues/391
Add a global flag to enable/disable privileged containers
Merge pull request #1288 from brendandburns/privilege

Add support for privileged containers.