start-kubemark.sh fails with unbound variable

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What happened:
Run test/kubemark/start-kubemark.sh, will get error:

k8s.io/kubernetes/test/kubemark/gce/../../../cluster/kubemark/gce/config-default.sh: line 32: TEST_CLUSTER_LOG_LEVEL: unbound variable

Try to fix this by export TEST_CLUSTER_LOG_LEVEL=--v=4 and rerun get another error:

k8s.io/kubernetes/test/kubemark/gce/util.sh: line 45: KUBE_GCE_INSTANCE_PREFIX: unbound variable

Basically, we should have default value for TEST_CLUSTER_LOG_LEVEL and KUBE_GCE_INSTANCE_PREFIX. I found that they have default values before, and was deleted accidentally in #78246. Although we can predefine these two variables before running the script, it will be more convenient to have default values, and more friendly to people who are not familiar with this script.

What you expected to happen:
We should be able to run test/kubemark/start-kubemark.sh without setting any environment variable manually.

How to reproduce it (as minimally and precisely as possible):
test/kubemark/start-kubemark.sh

/cc @wojtek-t @mm4tt @mborsz
/sig testing
/sig cluster-lifecycle
/assign

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I have opened a PR to fix this.