Increase max image depth to 127

Going from 42 to 127 which is a 3x increase.  127 is the hard coded AUFS default limit that is compiled with most aufs installs.

Two options for going over 127 is to recompile the aufs module or do this in addition to hard links.  The advantages of this approach is that it is all handled in aufs and nothing needs to be done outside of the core of aufs.

ping @shykes @jpetazzo