MB-8379 Always provide default value for num_threads parameter.

Change-Id: I2c15bf848eca8eb38b30f30a156e2032fe319d47
Reviewed-on: http://review.couchbase.org/26653
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>