CBD-792: add test configs for xdcr lag benchmarks

Change-Id: I0f9fa1e167ea9f27e61a93916f1b81054f4cf1ba
Reviewed-on: http://review.couchbase.org/24309
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>