NexthopGroup member change is not supported

[yue-fred-gao]

When running test_vxlan_ecmp sonic-mgmt test, I found nexthop group member change is not supported. This can happen in many situations. For this particular test case, a nexthop group member can be added or removed dynamically triggered by bfd state change. When nexthop group member changed, we need to delete the routes and reprogram them if they point to the nexthop group.