Automatic merge from submit-queue (batch tested with PRs 62510, 61689). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Extend kubectl describe for networkPolicy

**What this PR does / why we need it**:
Recently `networkpolicy` was allowed podSelector and namespaceSelector to be specified together in a NetworkPolicyPeer, so we need to extend `kubectl describe` for networkpolicy.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes kubernetes/kubectl#376

**Special notes for your reviewer**:
Here is the example output of `kubectl describe networkpolicies`:
NetworkPolicy file:
```yaml
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: test-network-policy
  namespace: default
spec:
  podSelector:
    matchLabels:
      role: db
  ingress:
  - from:
    - namespaceSelector:
        matchLabels:
          project: myproject
      podSelector:
        matchLabels:
          role: frontend
    ports:
    - protocol: TCP
      port: 6379
  egress:
  - to:
    - namespaceSelector:
        matchLabels:
          project: myproject
      podSelector:
        matchLabels:
          role: frontend
    ports:
    - protocol: TCP
      port: 6379
```
describe:
```shell
# ./kubectl describe netpol test-network-policy
Name:         test-network-policy
Namespace:    default
Created on:   2018-03-26 15:20:11 +0800 CST
Labels:       <none>
Annotations:  <none>
Spec:
  PodSelector:     role=db
  Allowing ingress traffic:
    To Port: 6379/TCP
    From NamespaceSelector: project=myproject
         PodSelector: role=frontend
  Allowing egress traffic:
    To Port: 6379/TCP
    To NamespaceSelector: project=myproject
       PodSelector: role=frontend
  Policy Types: Ingress, Egress

```

**Release note**:
```release-note
NONE
```