MB-11066: Temp: Change config change loglevel from INFO -> WARN

Config change log levels should conceptually be 'Info' level messages,
but we currently have the default level as 'WARN' (and the world logs
at that level - see MB-12035). For now go along with the rest of
memcached/ep-engine and use WARN.

Change-Id: I98f468ba7acb1d2908b842c1e75697ddc7e63661
Reviewed-on: http://review.couchbase.org/40754
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>