Allow local zone override for NonGCP NEG

[freehan]

This is for NonGCP where node label of fault domains may not be present.

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[cxhiano]

LGTM

Tested in a GKE cluster

[freehan]

/test pull-ingress-gce-test

[MrHohn]

/lgtm

[MrHohn]

/lgtm

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This pull-request has been approved by: freehan, MrHohn

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