Use pillar information for IP and gateway in MOTD

Summary:
Dwellers has a stale MOTD with legacy information
from Online. Probably best to templatize it, so we
can use up to date information from pillar.

Ref T1595.

Test Plan: Apply roles/core/motd on Equatower and Dwellers

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1595

Differential Revision: https://devcentral.nasqueron.org/D2217