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Default some unbound cluster/gce env vars

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

Sets defaults for two env vars used by cluster/gce/* scripts so as to
avoid the following warnings when bringing a cluster up for test
```
METADATA_CONCEALMENT_NO_FIREWALL: unbound variable
CUSTOM_KUBE_DASHBOARD_BANNER: unbound variable
```

**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 #60850

```release-note
NONE
```