Scale roles/shellserver/users/ to support several users account

Summary:
Currently, 62c123e has introduced users management for Eglide.

To allow this change to scale, we need to provide a set of users,
and run tasks against it (create account, add to group, add SSH key).

We so move users data in a 'shellusers' pillar, and add Jinja logic
to the state to use this dictionary.

Fixes T809.

Test Plan: salt eglide state.sls roles/shellserver/users

Reviewers: Sandlayth, dereckson

Maniphest Tasks: T809

Differential Revision: http://devcentral.nasqueron.org/D359