config.cc: add debug_ prefix to subsys logging levels

Add debug_ prefix also for 'ceph --admin-daemon *.asok config show'
as already done e.g. by 'ceph-osd --show-config'.

Fixes: #7602

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Reviewed-by: Sage Weil <sage@inktank.com>