Automatic merge from submit-queue
Cleanup pkg/kubectl
I was reading through `pkg/kubectl` in preparation for completing https://github.com/kubernetes/kubectl/issues/11 and noticed several opportunities for improvement. This should be easy to review since it's mostly mechanical changes. The only complicated changes are in `addFromEnvFile`, which I refactored into two functions and wrote tests for.
**Release note**:
```release-note
NONE
```