Add and validate namespace to pod, service, replication controller objects

Next iteration in delivering code against the integrated namespaces proposal, the PR adds the following:

1. Add Namespace to Pod, Service, ReplicationController objects
2. Enforces Namespace is set prior to persistence
3. Enforces context.Namespace on create and update operations matches resource.Namespace

Feedback welcome.

The long name was meant to re-enforce that the operation is really only relevant when doing a create or update operation, but I agree that is implied if you are passing in a resource as parameter to call.

Will update to just ValidNamespace.