Adopt salt-primary role name

Summary:
As the Salt project doesn't provide any name change,
let's adopt one: "Salt primary server" and "salt-primary" role.

Ysul and WindRiver MOTD aren't updated as development servers
and deployment servers are now separate entities, and that
role is deprecated for them.

A disclaimer is added for the match against the FreeBSD rc service.

Ref T1668

Test Plan: Deploy to Complector

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1668

Differential Revision: https://devcentral.nasqueron.org/D2641