group podLists into single list
group podLists into single list

Related Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1381621

When listing pods as JSON or YAML for one or more nodes with `oadm manage-node`, the
node name is printed to stderr, while the PodList object is printed to stdout.

This patch updates the output of `oadm manage-node --list-pods` to join the list of
pods of every node into a single PodList, allowing the output to consist of a single
api.PodList object.

**Before**
```
$ oadm manage-node --selector="type=infra" --list-pods -o json

Listing matched pods on node: ip-172-31-53-15.ec2.internal

{
    "metadata": {
        "selfLink": "/api/v1/pods",
        "resourceVersion": "350009"
    },
    "items": [
        {
....
}

Listing matched pods on node: ip-172-31-53-16.ec2.internal

{
    "metadata": {
        "selfLink": "/api/v1/pods",
        "resourceVersion": "350009"
    },
    "items": [
        {
...
}
```

**After**

`"items" will contain the pods of every node`
```
$ oadm manage-node --selector="type=infra" --list-pods -o json
{
    "metadata": {
        "selfLink": "/api/v1/pods",
        "resourceVersion": "350009"
    },
    "items": [
        {
...
}
```

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