This removes setting KUBE_GCE_PRIVATE_CLUSTER=false flag when creating
kubemark master.
In result, util.sh detect-master function detects both private and
public master IPs. The comment about cloud NAT does not apply after
https://github.com/kubernetes/kubernetes/pull/81073/files got merged
(see comments in the PR discussion).
This is first PR to change kubemark clusters to use private master IPs:
https://github.com/kubernetes/perf-tests/issues/874.
Note that kubemark kubeconfig will still contain public master IP. This
will be addresses in the follow-up PRs.
Testing:
* set up kubemark cluster
* verified that both private and public kubemark master IPs are logged
* ran tests on kubemark cluster using cluster loader
6f0eec4dff