Automatic merge from submit-queue (batch tested with PRs 54112, 54150, 53816, 54321, 54338). 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>.

Enable metadata concealment for tests

**What this PR does / why we need it**: Metadata concealment is going to beta for v1.9; enable it by default in tests.  Also, just use `ENABLE_METADATA_CONCEALMENT` instead of two different vars.  Work toward #8867.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: none

**Special notes for your reviewer**:

**Release note**:

```release-note
Metadata concealment on GCE is now controlled by the `ENABLE_METADATA_CONCEALMENT` env var.  See cluster/gce/config-default.sh for more info.
```