fix gnmi set issues and simplify interface

api/initialise_test.go

api/pnd.go

api/pnd_test.go

interfaces/change/change.go

interfaces/networkdomain/pnd.go

interfaces/transport/transport.go

mocks/NetworkDomain.go

mocks/Transport.go

northbound/server/pnd.go

northbound/server/pnd_test.go

nucleus/change.go

nucleus/change_test.go

nucleus/gnmi_transport.go

nucleus/gnmi_transport_test.go

nucleus/principalNetworkDomain.go

nucleus/principalNetworkDomain_test.go

test/integration/nucleusIntegration_test.go