MB-100: minor changes in _wait_warmup_completed logs

Change-Id: I2fe5e27dd26437b654a2bcb909e173a5a183e9da
Reviewed-on: http://review.couchbase.org/24197
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>