MB-100: Editing xdcrbasetests

- Setting paramenter "XDCR_OPTMISTIC_REPLICATION_THRESHOLD"
- Use input parameter 'optimistic_xdcr_threshold', default 256
- XDCR_LATENCY_OPTMIZATION parameter invalid

Change-Id: Id338f22b9cb9415c217d8198ada9b5cc2553717a
Reviewed-on: http://review.couchbase.org/25992
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>
Tested-by: abhinav dangeti <abhinav.dangeti@gmail.com>