network_build: use nemu to create net namespace, provide several network

re6st_wrap: wrap the deploy of re6st node, ease the creation of cert
file and run of the node

test_net contain the tests:
  - ping test: net segment(router), multi-net segment(demo),test after a
    reboot. Because net_demo contain a subnet UPnP, test is not stable.

ping is like ping file in demo, a script to run multiping