Update NetworkPolicy docs for v1

[danwinship]

Fixes #3934

@caseydavenport ok?

For 1.7 Features: set Milestone to 1.7 and Base Branch to release-1.7

I can't set the milestone...

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This change is 

[caseydavenport]

This LGTM.

@danwinship what do you think about a section which mentions how to maintain v1beta1 semantics across upgrades?

[danwinship]

I'd be fine with that although I'm not sure where upgrade-related stuff belongs in the fancy new docs organization... It's covered in the release notes already:

NetworkPolicy has been moved from `extensions/v1beta1` to the new
`networking.k8s.io/v1` API group. The structure remains unchanged from
the beta1 API.

The `net.beta.kubernetes.io/network-policy` annotation on Namespaces
to opt in to isolation has been removed. Instead, isolation is now
determined at a per-pod level, with pods being isolated if there is
any NetworkPolicy whose spec.podSelector targets them. Pods that are
targeted by NetworkPolicies accept traffic that is accepted by any of
the NetworkPolicies (and nothing else), and pods that are not targeted
by any NetworkPolicy accept all traffic by default.

Action Required:

When upgrading to Kubernetes 1.7 (and a network plugin that supports
the new NetworkPolicy v1 semantics), to ensure full behavioral
compatibility with v1beta1:

    1. In Namespaces that previously had the "DefaultDeny" annotation,
       you can create equivalent v1 semantics by creating a
       NetworkPolicy that matches all pods but does not allow any
       traffic:

           kind: NetworkPolicy
           apiVersion: networking.k8s.io/v1
           metadata:
             name: default-deny
           spec:
             podSelector:

       This will ensure that pods that aren't matched by any other
       NetworkPolicy will continue to be fully-isolated, as they were
       before.

    2. In Namespaces that previously did not have the "DefaultDeny"
       annotation, you should delete any existing NetworkPolicy
       objects. These would have had no effect before, but with v1
       semantics they might cause some traffic to be blocked that you
       didn't intend to be blocked.

[chenopis]

How to maintain v1beta1 semantics across upgrades could be a Task. Do you want to cover that in a separate PR so I can merge this one?