Navigator API allows resources with non-existent namespaces

[wallrj]

Prompted by a discussion with @munnerz on Slack:

We should have an admission controller that ensures the namespace within which a resource is being created actually exists If you create a resource that specifies a namespace on it, and that namespace doesn’t exist, there’s nothing to stop it being written to etcd right now (in Navigator). we need to check if a corresponding Namespace resource exists in the apiserver when things are created/during admission

/kind bug