Automatic merge from submit-queue (batch tested with PRs 63871, 63927, 63966, 63957, 63844). 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>.

kubectl: fix Flatten() when used without Latest()

**What this PR does / why we need it**:

If `Flatten()` is used on resource builder the list is not flattened unless the `Latest()` is used in the chain.
We should support `Flatten()` without Latest() as well. For example:

```
$ oc apply -l foo=bar -f /tmp/list.yaml
```
 will fail with:
```
F0517 13:45:07.831195   31795 helpers.go:119] error: object does not implement the Object interfaces
```

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