We rename it to EPHEMERAL_BLOCK_DEVICE_MAPPINGS, and we also change the value
so that it starts with a `,`, instead of always inserting a comma before it.
In this way the value can be empty.
Also, if the user sets the (currently experimental) KUBE_AWS_STORAGE
environment variable to be "ebs", then we will not mount any instance storage
which will cause the machines to use EBS storage instead.