Old node images are not being cleaned up when new ones are created

[boegel]

When a new node image (AMI) was created with sudo /usr/local/bin/run-packer, the old node image is left behind, even though it's not used anymore (at least in AWS it is).

When building a new node image was successful, the previous node image should be deleted (deregistered in AWS terms).

I guess this is related to #95 (perhaps even a duplicate? not sure...)