Add options for the filesystem used for devicemapper

This adds some env vars to allow you to control what filesystem docker uses for the device mapper devices.
DOCKER_FILESYSTEM: Controls what filesystem to use, atm ext4 and xfs is supported
DOCKER_MKFS_ARGS: Lets you pass extra arguments to mkfs that creates the base device
DOCKER_FS_OPTIONS: Allows you to pass extra runtime mount options for the thin devices.