Only impose 42-layer MaxImageDepth if using AUFS.

Presently, we're unable to stack more than 42 AUFS layers and docker will raise an error if this limit is reached. However, this error is raised even if docker is not using the AUFS graphdriver.

This commit solves the problem by ensuring that the ```MaxImageDepth```, now ```AUFSMaxImageDepth```, is only checked if docker is using the AUFS graphdriver.