Make use of /usr/sbin/ebslvm

The new baseami provides /usr/sbin/ebslvm which we use
here to mount ebs volumes in an lvm in /mnt, that means
we no longer provide the symlink during the build phase
Oh, I think it should absolutely fail if EBS is not available. I am suggesting that if this script is run again (say, after OOM killer or something wacky) that it will fail before launching our application.