Configure Notifications center

Summary:
Currently, the notifications center configuration is stored into
an ad hoc repository, operations-data-notifications (rOPSDATAN).

Configuration files are JSON, which can be more compactly expressed
and maintained in YAML, then dumped on the server as JSON.

As we need to provision those files and set SELinux context for them,
they can be hosted directly on rOPS too.

Secrets for credentials.json are stored in Vault.

Test Plan: Deploy container and config on docker-001.

Reviewers: dereckson, DorianWinty

Reviewed By: DorianWinty

Tags: Nasqueron Docker deployment squad, Notifications center

Differential Revision: https://devcentral.nasqueron.org/D2354