CBQE-2249: use_channel=True for execute_cbtransfer

Change-Id: Iece5ef71ddc472dccb8377ff79772d16b881903b
Reviewed-on: http://review.couchbase.org/37035
Reviewed-by: Parag Agarwal <agarwal.parag@gmail.com>
Tested-by: Parag Agarwal <agarwal.parag@gmail.com>