fix load balancer regstration/de-registering within ec2 vpc

rolling_deploy with load_balanced:true fails with:
[Capify-EC2] Deployment aborted due to error: undefined method `id' for nil:NilClass!
because no instances are found (the dns name doesn't match the IP address).
Changing to use instance.contact_point fixes this, and correctly registers/deregisters VPC instances.