fix deploy open5gs-compose #547

[PapaySail]

Corrections in the manual and in docker-compose.ue2.yaml

• removed iptables flush and config
• added sudo apt install iperf3
• added command to turn off tx offload @ new bridge interface
• fixed env IPERF_HOST: 172.20.0.1
• fixed env GNB_HOSTNAME: open5gs-compose-gnb-1

[linouxis9]

To be noted, I also had to disable tx offloading when using eUPF's eBPF inside PacketRusher for TCP to work (else packets were rejected with invalid checkums). Though it's weird to me for it to be necessary as the eBPF does seem to fix the checkum of the packet (eg. https://github.com/edgecomllc/eupf/blob/ec283f5c857cd81a05c47d643c49c84adc4df41f/cmd/ebpf/xdp/utils/gtp_utils.h#L235)