Handle case where no instances are returned after calling AWS RunInstances

In certain unpredictable cases calling RunInstances can return an empty list of instances. This case isn't handled by docker-machine but is by deploykit - https://github.com/docker/deploykit/blob/master/pkg/provider/aws/plugin/instance/ec2_instance.go#L272 . The docker-machine issue is open here https://github.com/docker/machine/issues/4746

In these cases instances do end up created but not tracked and so cannot be cleaned up leaving them to incur usage costs but provide no benefit.

Closes #6 (closed)