Propagate pillar values to config.json for Phabricator containers

Summary:
Some values used in config.json are already declared in pillar.
This change reuses them, so we've less hardcoded values.

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/D3729