cephadm: enable logging to stderr in ceph-iscsi

cephadm: enable logging to stderr in ceph-iscsi

All the other ceph servies when deployed are deployed with:

  --default-log-to-stderr=true --default-log-stderr-prefix=debug
  --default-log-to-file=false

Ceph-iscsi doesn't have these as commandline params but once
https://github.com/ceph/ceph-iscsi/pull/186 lands it'll have something
similar as config options. This patch sets these options

Signed-off-by: Matthew Oliver <moliver@suse.com>
(cherry picked from commit f65584637f8ee44e79e0976dcecc086d2ba7fe45)