Fix alias_evaluate_target_health bug by setting it to boolean

This pull request fixes the bug introduced in boto/boto#2120, reported by @cbrinker.
- `alias_evaluate_target_health` attribute is now a boolean. It is consistent with the `add_change()`, which takes a boolean value as `alias_evaluate_target_health` parameter.
- Unit tests were updated with both `EvaluateTargetHealth` true and false cases.