ceph-disk: support and test the absence of PATH

ceph-disk: support and test the absence of PATH

Although this is not exactly the context in which ceph-disk is run when
invoked by udev, it makes sure there is at least one sensible way of
using it when PATH is undefined.

Also make src/ceph.in not fail if PATH is not defined.

Signed-off-by: Loic Dachary <loic@dachary.org>