MB-8663 Let users choose between xmem/capi replications.

I just added 'type' parameter to the ones understood by
createReplication REST endpoint. Valid values are capi and xmem. They
translate to type=xdc and type=xdc-xmem in replicator document
correspondingly.

Change-Id: Iacacd51d7525cf9c66259bcce72037eedd1667cd
Reviewed-on: http://review.couchbase.org/28170
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>