Merge PR #27278 into nautilus

* refs/pull/27278/head:
	log,global: do not start flusher thread until after we have our mon config
	log: buffer log entries until flusher thread starts
	log: open log file from flusher thread
	common/ceph_context: fix log_to_file observer
	common: add bool log_to_file option

Reviewed-by: Kefu Chai <kchai@redhat.com>
Reviewed-by: Neha Ojha <nojha@redhat.com>