Expand Istio property replica counts to avoid problems when draining Istio nodes

When draining Istio pods some nodes may not drain because they have a replica count of 1, while 3 or more replicas are required. In order to avoid this, you should set the minimum replica count for each property to at least 3.

When backing up and draining Istio nodes you may see errors like: 

kubectl drain` failed to evict and reschedule pods to another node due to PodDisruptionBudget definitions:
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NAMESPACE NAME MIN AVAILABLE MAX UNAVAILABLE ALLOWED DISRUPTIONS AGE
istio-system istio-ingressgateway 1 N/A 2 5d11h
istio-system istio-pilot 1 N/A 2 5d11h
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For istio-ingressgateway and istio-pilot you can set the istio.ingressgateway.replicaCountMin and istio.pilot.replicaCountMin properties to 3 or higher. See the Configuration properties reference, istio.

For other istio components (that don't have a replicaCountMin property), see Manually upgrading a cluster or node pool in the GKE documentation or IstioOperator Options in the Istio documentation.