Provide MySQL credentials template file to ViperServ

Summary:
As Salt can't currently manage credentials, we provide
a template file with a die instruction explaining how
to fill it.

Ref T1309

Test Plan: Daeghrefn has successfully been connected to MySQL.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1309

Differential Revision: https://devcentral.nasqueron.org/D1214