Bucket Priority default value is 8

Change-Id: Ib512e1b5ef5350360e7a86b20ffb9dadb4e833b4
Reviewed-on: http://review.couchbase.org/31467
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>