Align MIG update policy with GKE (maxSurge=1 maxUnavailable=0)

[jeffmccune]

The update policy of max_unavailable_fixed = 1 could result in capacity being reduced during a rolling update, causing disruption. If the autoscaler has a target size of 6, then one instance may be removed while the replacement instance is still in the process of being created.

The default behavior should align with GKE, see Determining your optimal surge configuration

All new node pools are automatically configured to use surge upgrades (maxSurge=1 maxUnavailable=0).