Fix the AZ name to AZ ID mapping in tests.

[orsenthil]

Fix the AZ name to AZ ID mapping in tests.

• Don't rely on node index, which is an independent entity.

There is a mistake in the code, which node index to get the AZ name to AZ ID mapping. This works only by accident.

If we have 3+ nodes in a cluster in us-west-1 region, this will fail as the us-west-1 has only two default AZs.

$ aws ec2 describe-availability-zones --region us-west-1
{
    "AvailabilityZones": [
        {
            "State": "available",
            "OptInStatus": "opt-in-not-required",
            "Messages": [],
            "RegionName": "us-west-1",
            "ZoneName": "us-west-1a",
            "ZoneId": "usw1-az1",
            "GroupName": "us-west-1",
            "NetworkBorderGroup": "us-west-1",
            "ZoneType": "availability-zone"
        },
        {
            "State": "available",
            "OptInStatus": "opt-in-not-required",
            "Messages": [],
            "RegionName": "us-west-1",
            "ZoneName": "us-west-1c",
            "ZoneId": "usw1-az3",
            "GroupName": "us-west-1",
            "NetworkBorderGroup": "us-west-1",
            "ZoneType": "availability-zone"
        }
    ]
}

This failure was observed in internal tests, and PR fixes this.

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