fix gRPC timing issue - #156

[youngtaekiim]

nodeagent

remove unwrap() in gRPC sender and do not send any container list if gRPC server is not available.

[ec2-user@ip-172-31-4-38 src]$ sudo podman logs -f piccolo-agent-nodeagent
Starting NodeAgent on host: HPC
NodeAgentManager init
NodeAgentManager successfully initialized
NodeAgent listening on 0.0.0.0:47004

thread 'tokio-runtime-worker' panicked at nodeagent/src/grpc/sender.rs:38:18:
called Result::unwrap() on an Err value: tonic::transport::Error(Transport, ConnectError(ConnectError("tcp connect error", Os { code: 111, kind: ConnectionRefused, message: "Connection refused" })))
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
NodeAgentManager stopped

filtergateway

If filtergateway cannot get any scenario info from etcd, return empty scenario instead of Err()

[ec2-user@ip-172-31-4-38 yaml]$ sudo podman logs -f piccolo-player-filtergateway
FilterGatewayManager init
Piccolod gateway listening on 0.0.0.0:47002
Failed to initialize FilterGatewayManager: GRpcStatus(Status { code: Unavailable, message: "tcp connect error: Connection refused (os error 111)", source: Some(tonic::transport::Error(Transport, ConnectError(ConnectError("tcp connect error", Os { code: 111, kind: ConnectionRefused, message: "Connection refused" })))) })