cephadm: avoid crashing on expected non-zero exit

cephadm: avoid crashing on expected non-zero exit

- Avoid crashing when a call out to an external program expectedly does
  not return exit status zero.

There are programs that communicate other information than error/no
error through exit status. E.g. `systemctl status` will return different
exit codes depending on the actual status of the units in question.
In cases where this is expected crashing with a RuntimeError exception
is inappropriate and should be avoided.

Fixes: https://tracker.ceph.com/issues/55117
Signed-off-by: Moritz Röhrich <moritz.rohrich@suse.com>