global: call config observers on global_init (and start logging!)

global: call config observers on global_init (and start logging!)

Currently we don't start logging on daemon startup unless the log_file
parameter was adjusted by ceph.conf.  Instead, we should call all config
observers so that the logging subsystem is fully configured and we log
even prior to the daemonize and common_init_finish (when we call observers
again).  This fixes logging for the initial period before we daemonize.
For some of the daemons (osd, mon), this includes significant work.  It
also fixes the problem where users don't see the 'ceph version ...' banner
on daemon start.

Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>