cephadm: fix logging defaults

cephadm: fix logging defaults

- prefix stderr log lines with 'debug '
- send cluster log to stderr (only affects mons)
- default to sending cluser log to stderr only (not to file)

This aligns things with the container-y way, as we do
with rook.  It means that if you want legacy behavior (log files),
you have *2* settings to change:

 log_to_file = true
 mon_cluster_log_to_file = true

We could default this to leave cluster log to file by default
since they aren't that big (and we do have log rotation), but
it's not as clean as it could be...

Signed-off-by: Sage Weil <sage@redhat.com>