AWS: Configure LoadBalancer health checks

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.

pkg/cloudprovider/providers/aws/aws_test.go

@@ -430,6 +430,10 @@ func (ec2 *FakeELB) ApplySecurityGroupsToLoadBalancer(*elb.ApplySecurityGroupsTo
 	panic("Not implemented")
 }
 
+func (elb *FakeELB) ConfigureHealthCheck(*elb.ConfigureHealthCheckInput) (*elb.ConfigureHealthCheckOutput, error) {
+	panic("Not implemented")
+}
+
 type FakeASG struct {
 	aws *FakeAWSServices
 }