Help to configure Salt for Vault access on shellserver

Summary:
The procedure is documented in the Operations grimoire at
https://agora.nasqueron.org/Operations_grimoire/Eglide/Vault

This change helps to prepare the configuration file,
but role_id and secret_id needs to be put manually.

Ref T1890

Test Plan: Tested on Eglide

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1890

Differential Revision: https://devcentral.nasqueron.org/D3154