Provision MediaWiki SaaS entry point

Summary:
The MediaWiki SaaS entry point is now stabilized enough to be
deployed directly by Salt.

Switch databases to db-B cluster.

Credentials are stored in Vault and provisioned through Salt.

Ref T845

Test Plan: Deploy on db-B-001 and web-001

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T845

Differential Revision: https://devcentral.nasqueron.org/D3025