Generalize DevCentral configuration

Summary:
The configuration has been prepared for devcentral container.
This change prepares reuse for other instances too.

The final target configuration file doesn't change, only the way
we declare data in pillar and pass it to the template.

Test Plan: Salt operation is no-op for /srv/phabricator/devcentral/conf/local/local.json

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3728