fix containerd start failed when config.toml already has other proxy_plugins

[lijianfeng1993]

when containerd config.toml has other proxy_plugins,the [proxy_plugins] will conflict with nydus config

      [proxy_plugins]
        [proxy_plugins.nydus]
          type = "snapshot"
          address = "/run/containerd-nydus/containerd-nydus-grpc.sock"

so containerd cannot restart success

[gaius-qi]

@lijianfeng1993 Please fix lint.