Fix UT failures

UT was failing for the following reasons
-In Oper Interface Add path the GetOSParams API used to invoke get_table() which returns NULL. The table object is used to fetch agent object
-In KSync Nexthop delete path while setting setting Encap info the vhost_interface object accessed from Agent object was NULL.

Fix
-Pass agent object to GetOSParams in Add path.
-For Nexthop delete vrouter does not need anything apart from NH-index. Don't encode anything apart from NH-id in NH delete request msg.