Create kubemark cluster as private

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:
kubernetes/perf-tests#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

Author: Jakub Pierewoj

test/kubemark/gce/util.sh

@@ -44,9 +44,6 @@ function create-kubemark-master {
     export KUBE_CREATE_NODES=false
     export KUBE_GCE_INSTANCE_PREFIX="${KUBE_GCE_INSTANCE_PREFIX}-kubemark"
 
-    # Even if the "real cluster" is private, we shouldn't manage cloud nat.
-    export KUBE_GCE_PRIVATE_CLUSTER=false
-
     # Quite tricky cidr setup: we set KUBE_GCE_ENABLE_IP_ALIASES=true to avoid creating
     # cloud routes and RangeAllocator to assign cidrs by kube-controller-manager.
     export KUBE_GCE_ENABLE_IP_ALIASES=true
@@ -94,8 +91,6 @@ function delete-kubemark-master {
     export KUBE_GCE_INSTANCE_PREFIX="${KUBE_GCE_INSTANCE_PREFIX}-kubemark"
 
     export KUBE_DELETE_NETWORK=false
-    # Even if the "real cluster" is private, we shouldn't manage cloud nat.
-    export KUBE_GCE_PRIVATE_CLUSTER=false
 
     if [[ "${KUBEMARK_HA_MASTER:-}" == "true" && -n "${KUBEMARK_MASTER_ADDITIONAL_ZONES:-}" ]]; then
       for KUBE_GCE_ZONE in ${KUBEMARK_MASTER_ADDITIONAL_ZONES}; do