Fix ephemeral mounts

Add all possible ephemeral block mounts during AMI registration

Value for this being if not using directly AWS to launch a server (ergo skipping the assignment).

Prime case is Rightscale: they create their images with 1-4 ephemeral mounts. Due to this on some instance sizes you don't see all the available ephemeral storage and there is no way to add it afterwards.
Instead of doing a big if/else you could just take create an argument list and append the `--block-device-mapping` param if needed and then call `euca-bundle-image` with that argument list.