Cleanup python-level constructors

Change-Id: I831b48cf4d9f5d3439817b45fbac9c597a4213eb
Reviewed-on: http://review.couchbase.org/26551
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>