Fixing aws_elb schema

If not suppling the `availability_zones`, they will be computed
(meaning an update/refresh will retrieve the info and update the values
to the state file).

So without the `Computed = true` the diff will always flag this as a
change, even when it’s not.
Merge pull request #716 from svanharmelen/f-provider/aws-fix-aws-elb

provider/aws: fixing aws_elb schema