ceph-disk: allow using a regular file as a journal

ceph-disk: allow using a regular file as a journal

Because of a missing return, ceph-disk prepare would fail if given a
regular file as a journal. If the journal file does not exist, ceph-disk
will create it but fail to ensure that the ceph user owns it. The
symlink to the journal file is not set when the journal file is
specified on the command line and the journal file does not exist at
all. The ceph-osd daemon will silently create it as a file but it will
not be the file given in argument.

Add a test case to verify using a regular file as a journal works as
expected.

Fixes: http://tracker.ceph.com/issues/17662
Signed-off-by: Jayashree Candadai <jayaajay@indiana.edu>
Signed-off-by: Loic Dachary <ldachary@redhat.com>
(cherry picked from commit db917d50eb5f86a07a5487e130f46a6b1d27672a)