Resolve conflict for Salt Vault configuration

Summary:
Both core and shellserver roles contains the state
of the Salt configuration file for Vault access.

On shellserver role, a dedicated Vault service
is run for local credentials.

Fixes T1998.

Test Plan: Deploy on Eglide roles/core/salt

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1998

Differential Revision: https://devcentral.nasqueron.org/D3401