Updating the resource to use a set instead of a list

By using a set for the availability zones, you can use things like
`availability_zones = ["${aws_instance.web.*.availability_zone}"]`
where is very likely multiple of the same zones will be added to the
set. If you use a list here, the list will say it’s changed (even if
you add the same zone) which will force a new resource.
Merge pull request #680 from svanharmelen/f-provider/aws-update-aws-elb-resource

provider/aws: updating the resource to use a set instead of a list