network: fix agent secret filling issue

- enhance secret filling rules for 8021x, pppoe and vpn
- refactor code, get vpn information through parseing
  name files

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/bbd4cc5f1d9141d19197f04a352768c5/
Change-Id: I056bf64bbf3e13d4e948e0fcddd124bba1f3c275