1. What is the problem
In the current l3 networking model, the host route will be only valid
after DHCP lease time expired, and renewed.
2. What is the solution for the problem
To overcome the drawbacks, a new l3 networking model is introduced in
this spec(https://review.openstack.org/#/c/530904/).
This part of implement includes bottom external network creation after
segment-create and bottom external subnet creation after subnet-create
using segment.
3. What features need to be implemented to the Tricircle to
realize the solution
N/A
[Test record]http://note.youdao.com/noteshare?id=e2e5e4bf6e73db1bc55f47963a72b39d&sub=6158D53B085D4A7B8EB046F01FB62748
Change-Id: I7d968a402b23428b4d3a7061ec507ed30a6132ef