Provide execution module for Zemke-Rhyne

Summary:
The cachet container uses Zemke-Rhyne to get credentials.

This change moves command building logic to a proper execution module.

Test Plan:

• salt equatower zr.get_username 124
• salt equatower zr.get_password 124

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: http://devcentral.nasqueron.org/D1718