ELB will automatically create a health check, but if we update the
listeners the old health check port sticks around, and all the instances
are marked offline.
Update the health-checks to match the listeners: we just check the first
valid service port, with some hard-coded options for timeouts / retries etc.
This turned out to be a little convoluted, but is needed because deleting an ELB on AWS
is a painful UX - it won't have the same endpoint when it is recreated.
Also started splitting the provider into files, but only for new functions (so far!)
Previously the servicecontroller would do the delete, but by having the cloudprovider
take that task on, we can later remove it from the servicecontroller, and the
cloudprovider can do something more efficient.
