Split IPv6 network configuration

Summary:
Fixes the following issue:

Rendering SLS 'base:roles/core/network.ipv6' failed: did not find expected <document start>
in "<unicode string>", line 90, column 1

Test Plan: salt web-001 state.apply roles/core/network test=True

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3384