Simplify webserver-content management

Summary:
With Salt Tower, we can define a different content for a pillar value
per node.

That allows to avoid any need to filter by role or to maintain an index.

The pillar contains the list of roles by node, and Salt will only
provision those.

Furthermore, this fixes issues where:

• pillar variables defined only for a specific role was needed
• it tries to fetch Vault secrets the node doesn't have access to

Test Plan: Deploy to devserver, shellserver, webserver-alkane

Reviewers: DorianWinty, ledesillusionniste

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3334