From: Matt Helsley <matthltc@us.ibm.com>

If lxc-debian fails or is interrupted during debootstrap then the next
invocation of lxc-debian breaks because it only checks for the existence
of the directory. This forces the user to remove the cache by hand to
retry the create step.

Let's allow the user to re-run lxc-debian to resume/retry. Store the
cache in a partial-$ARCH directory until debootstrap succeeds. Then move
the valid cache to its final destination.
Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>