Support for mongodb replicaset connection (HA).

Added replicaset field in nbi.cfg file, which is used in
mongodb connection string (osm_common/db_mongo.py).

Change-Id: I5b4c15463a6593319f2d9f097b41a3bb9688fcc6
Signed-off-by: Vijay Nag B S <vijaynag.bs@tataelxsi.co.in>