Configure IPv6 with DUID for Online network

Summary:
Online requests DHCP connection with a specific DUID.

It's treated as a secret to restrict range use to a specific account.
As such, it's stored in Vault, hence a policy change and specialized
credentials methods.

Ref T1861.

Test Plan: Tested on Windriver

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1861

Differential Revision: https://devcentral.nasqueron.org/D3185