sandbox-ora: Add SERVICE_VARIANT to xqueue to customize filenames

To be used in conjonction with
https://github.com/antoviaque/xqueue/commit/303601ea9ffcbd30f6bb3e2b9a42d6f85fbcf6b3

This is necessary to be able to get several services using the
`/opt/wwc/env.json` & `/opt/wwc/auth.json` files. The configuration
files would get overwitten when this is used by several different
services cohabiting on a single instance
sandbox-ora: Fixes `NaN` in config when nb of cores is incorrectly detected
sandbox-ora: Add missing '/' in configuration of open ended interface
sandbox-ora: Switch to HTTP for contacting Xqueue to match LMS defaults
sandbox-ora: Allow to configure Rabbitmq binding IP & use localhost for sandbox
sandbox-ora: Create edx user expected by xqueue in rabbitmq