ceph-disk: rely on /dev/disk/by-partuuid instead of special-casing journal symlinks

This was necessary when ceph-disk-udev didn't create the by-partuuid (and
other) symlinks for us, but now it is fragile and error-prone.  (It also
appears to be broken on a certain customer RHEL VM.)  See
d7f7d613512fe39ec883e11d201793c75ee05db1.

Instead, just use the by-partuuid symlinks that we spent all that ugly
effort generating.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit 64379e701b3ed862c05f156539506d3382f77aa8)