UBUNTU: SAUCE: shiftfs: enable overlayfs on shiftfs

UBUNTU: SAUCE: shiftfs: enable overlayfs on shiftfs

BugLink: https://bugs.launchpad.net/bugs/1838677
This patch enables overlayfs to use shiftfs as an underlay.

Currently it is not possible to use overlayfs on top of shiftfs. This
means Docker inside of LXD cannot make user of the overlay2 graph driver
which is blocking users such as Travis from making use of it
efficiently.

Co-Developed-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@canonical.com>