Allow more than 26 disks per instance

Following cosmos2d, we should adapt the code here to allow more than 26 disks:

    # Convert numeric disk slot index to device letter, so
    # 0 becomes a, 1 becomes b, etc.
    diskLetter=$(printf \\$(printf '%03o' $((97 + $diskSlot))))
    device="/dev/xvd${diskLetter}1"

This needs to be extended to use two letters when the index is greater than 25.