Map shells by OS

Summary:
Currently, the users list provide absolute, linux-centric paths
to the shell. Instead, we store only now the shell name, and
provide a map to resolve this shell name by OS.

Signed-off-by: Sébastien Santoro <dereckson@espace-win.org>

Test Plan: Apply roles/shellserver/users state, check no shell change.

Reviewers: xcombelle, dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D974