Use osfinger Salt grain instead of manually update MOTD

Summary:
Most MOTD offers OS / distro information, the same actually Salt
offers with the osfinger grain.

Test Plan: salt '*' state.apply roles/core/motd

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3292