Refactor and consolidate Zemke-Rhyne unit

Summary:
Strangely, Zemke-Rhyne was associated to the webserver-legacy role.

Actually, Zemke-Rhyne is mainly used to provision secrets
to Docker containers. So it's more logic to deploy it to
the Saltmaster role, as it's an integral part of deployment.

The .ssh/authorized_keys file will be reprovisonned each time
configuration (and so data/servers.json) changes.

Ref T1412

Test Plan: salt --local state.apply roles/saltmaster/zemke-rhyne

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1412

Differential Revision: http://devcentral.nasqueron.org/D1689