Deploy Salt Tower on salt-primary role

Summary:
Software and Salt configuration needed by D2862 refactoring.

See this change for the context and rationale.

Ref T1768.

Test Plan: Deploy and test D2862 change

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1768

Differential Revision: https://devcentral.nasqueron.org/D2869