Move shells provisionning to core role

Summary:
Currently, the core/users unit creates user accounts with specific shells
(zsh for example). This means we need to have those shells installed as
a part of the core and not only for dev/shell servers.

Test Plan: Check zsh installs correctly on Dwellers

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1379