enable nested virtualization in prow GCP tests

[medyagh]

currently

the prow kvm test use (32 cores)
n2-standard-32

while our internal gcp project (old infra) uses
n2-standard-8 (8 vCPUs, 32 GB Memory)

but our old infra uses special image that supports nested virtuazliation
(as explained here http://www.brianlinkletter.com/enable-nested-virtualization-on-google-cloud/)

the prow tests despite having much more cores are running signifancly slower
as seen in these two gopoghs (Sort by Duration)

Prow:
https://storage.googleapis.com/kubernetes-ci-logs/pr-logs/pull/minikube/21556/integration-kvm-docker-linux-x86-64/1982572213521354752/artifacts/test.html

minikube internal gcp project:
https://storage.googleapis.com/minikube-builds/logs/master/42124/KVM_Linux.html

Example Prow:
TestKubernetesUpgrade | 676.92 seconds
Example Minikube Jenkins:
TestKubernetesUpgrade | 208.78

(check full gopogh links above and sort by duraiton)

[medyagh]

in the PR we set the timeout for 60 minutes (while it used to be 5 minutes)

	ctx, cancel := context.WithTimeout(context.Background(), Minutes(60))

how about we try limit the paralleisim of thest

[medyagh]