Forbid creating clusters with more than 100 nodes without vpc-native

[marseel]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Currently GCE cannot support large clusters without vpc-native. Therefore this PR is adding check to make sure nobody is trying to create such cluster.
Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:
/sig scalability
/cc @mborsz
/cc @wojtek-t

[k8s-ci-robot]

@marseel: This issue is currently awaiting triage.

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[marseel]

/assign @wojtek-t

[marseel]

/assign @wojtek-t

[wojtek-t]

nit: can you change that int:

if [ "${ENABLE_IP_ALIASES}" = true ]; then
 ...
else
  if [[ -n "${MAX_PODS_PER_NODE:-}" ]]; then
    ..
  fi
  if [[ <your condition>]]; then
    ..
  fi
fi

It's easier to follow that those conditions are basically not really connected.

[marseel]

Makes sense, done.

[wojtek-t]

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: marseel, wojtek-t

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Needs approval from an approver in each of these files:

• cluster/gce/OWNERS [wojtek-t]

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[mborsz]

/retest

[marseel]

/retest

[marseel]

I've only rebased this branch, because test jobs were exiting with error "merge: 560397e0600a7ef36995b6c097e8ca5d1c54850a - not something we can merge"

[wojtek-t]

/lgtm
/retest