New unit for shellserver: users

Summary:
We want to manage users accounts through Salt, to automate account
provisionning and allow to restore them if an incident occurs.

Signed-off-by: Yassine Hadj Messaoud <modepadu95@riseup.net>

Test Plan: Already tested on Eglide

Reviewers: Sandlayth

Differential Revision: http://devcentral.nasqueron.org/D353