Read EvaluateTargetHealth from resource record set

This change will allow boto to read `<EvaluateTargetHealth>` tag from AWS response and set either `true` or `false` correctly.

Currently `alias_evaluate_target_health` is always `None`, even if the response from AWS contains:
```
"AliasTarget": {
    "HostedZoneId": "ABCDEFG12345",
    "EvaluateTargetHealth": true,
    "DNSName": "abcdefg-12345.us-west-2.elb.amazonaws.com."
}
```