Automatic merge from submit-queue (batch tested with PRs 60011, 59256, 59293, 60328, 60367). 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>.

Serve OpenAPI spec with single /openapi/v2 endpoint

**What this PR does / why we need it**:
We are deprecating format-separated endpoints (`/swagger.json`, `/swagger-2.0.0.json`, `/swagger-2.0.0.pb-v1`, `/swagger-2.0.0.pb-v1.gz`) for OpenAPI spec, and switching to a single `/openapi/v2` endpoint in Kubernetes 1.10. The design doc and deprecation process are tracked at: https://docs.google.com/document/d/19lEqE9lc4yHJ3WJAJxS_G7TcORIJXGHyq3wpwcH28nU

Requested format is specified by setting HTTP headers

header | possible values
-- | --
Accept | `application/json`, `application/com.github.proto-openapi.spec.v2@v1.0+protobuf`
Accept-Encoding | `gzip`

This PR changes dynamic_client (and kubectl as a result) to use the new endpoint. The old endpoints will remain in 1.10 and 1.11, and get removed in 1.12. 

**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 #

**Special notes for your reviewer**:

**Release note**:

```release-note
action required: Deprecate format-separated endpoints for OpenAPI spec. Please use single `/openapi/v2` endpoint instead.
```

/sig api-machinery