Make sink type part of sink module definition.

Change-Id: I15db21c35c70fbad5b0857b102ebb07a7b97b482
Reviewed-on: http://review.couchbase.org/37745
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>