Support --storage-opt for aufs graph driver

This patch alllows user to pass extra mount options to aufs graph driver via the daemon's `--storage-opt` flag. 

Our use case is that we want to test `dirperm1` mount option ( http://aufs.sourceforge.net/aufs3/man.html ) which only available to kernel with newer aufs patches to see if it can mitigate the permission bug with aufs (https://github.com/docker/docker/issues/783)
I believe this was the whole 42 layers problem so after awhile the size was too long and that's why everyone thought the limit was 42 layers when really we can out of bytes
See https://github.com/docker/docker/pull/9151