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Fixes kubectl Pod Disruption Budget and adds tests

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

This PR fixes several kubectl Pod Disruption Budget issues:
* The `min-available` parameter in Pod Disruption Budget V1 is not required (otherwise it will never use the default value)
* Removes the deprecated `min-available` default in Pod Disruption Budget V2
* The `selector` parameter in Pod Disruption Budget V2 is required
* Fixes (typo) the `max-unavailable` parameter check in Pod Disruption Budget V2
* Fixes some  assertion error messages where the value printed was always the zero value of the parameter type instead of the parameter provided
* Updated kubectl Pod Disruption Budget to use policy V1Beta1 instead of unversioned API (see https://github.com/kubernetes/kubectl/issues/90)
* Add missing tests

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes https://github.com/kubernetes/kubectl/issues/101

**Special notes for your reviewer**:

Spllited in several commits to make the review process easier. Please let me know if you prefer to squash some commits or if you prefer to split them in different PRs.

**Release note**:

```release-note
NONE
```

/cc @kubernetes/sig-cli-pr-reviews