Salt configuration for IPv6 for Debian

Summary:
On Debian, network could be managed putting files
in /etc/network/interfaces.d.

Ubuntu follows also this system.

Resolves T768.

Test Plan:
Run on the master:

salt eglide state.sls roles/core/network/init

Reviewers: Sandlayth, dereckson

Subscribers: dereckson, Sandlayth

Tags: IPv6

Maniphest Tasks: T768

Differential Revision: http://devcentral.nasqueron.org/D370