Fixing the flipflop problem

Actually meant as a PoC, but it works perfectly so let’s just merge
this…
Finishing up the needed changes

Also executed the acceptance tests successfully now.
Updated the code so it makes use of the available functionality

The `helper/schema` package already knows how to create a set from a
slice with set items. So let’s use that functionality…
Merge pull request #677 from svanharmelen/f-provider-aws-fix-route53-toggling

provider/aws: fixing the flipflop problem for the aws_route53 resource