1. What is the problem
Flat network type is commonly used as the external network type, but
currently users can not create a flat external network via Tricircle.
2. What is the solution for the problem
Support flat network type.
3. What features need to be implemented to the Tricircle to
realize the solution
(1) A new type driver for flat network is added.
(2) Release note is added
(3) Related documents are updated
Change-Id: I148e1102510dda96a9fcd8a4b76de09cd802833c
ee008cae6b