Only takes the first available subnet in a AZ, ignore other subnets
and log warning about this.
Removes AWS region comparison for subnet AZs. A VPC is only in a single
AWS region.
Fixes#12381
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!)
Christian Simon