Add .log suffix to log files.

Change-Id: Ia43f471a9c2c23afc0cc46aedf14236da7d5be10
Reviewed-on: http://review.couchbase.org/37750
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>