Automatic merge from submit-queue
Fix GKE kube-up to correctly find an IGM from a multi-zone cluster
I've confirmed that this successfully brings up a cluster, fixing the immediate issue with the new e2e test. Sorry about not properly vetting it in the original PR (#24075).
This does cause a warning message to be printed based on the handling of the NUM_NODES variable though, which I could fix if you guys think it's worth it:
```
Detected 6 ready nodes, found 6 nodes out of expected 3. Found more nodes than expected, your cluster may not behave correctly.
```
@quinton-hoole
1186f4bf85