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.
[There's an easier way you know...](https://github.com/andsens/bootstrap-vz/blob/02c683120aa1cdd426b10e7bd199acde20ad5aee/bootstrapvz/providers/ec2/ebsvolume.py#L27) :-)