config: don't make noise about 'internal_safe_to_start_threads'

This is set on start, and subsequently gets into the changed set.
Once any other config value is injected, it is the first thing reported
by the logs, but is confusing and useless to the user.  Hide it.

Signed-off-by: Sage Weil <sage@inktank.com>
config: clean up output

Report a simple list of key='value', without extra verbosity.

Signed-off-by: Sage Weil <sage@inktank.com>
config: report on log level changes

Signed-off-by: Sage Weil <sage@inktank.com>
Merge branch 'wip-config'

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>