spec: define pod security requirements

[jonboulle]

The spec needs to touch on what the security requirements are for a pod.

Is any isolation of users required between pods?
What user does each application run under and can this be root (i.e. "real" root in the host)?

This might (but not necessarily) be addressed by #231

[philips]

I think it can be any user including root. Isolation between pods seems like an executor and configuration and it feels out of scope for the spec.