config-prefix: Allow to specify SERVICE_VARIANT, used as config prefix

Several services are currently using the same configuration filenames `/opt/wwc/env.json` & `/opt/wwc/auth.json` files. The configuration files would get overwritten when this is used by several different services cohabiting on a single instance. This patch allows to use the same convention as on the LMS/CMS (role edxapp on the configuration repository) to prefix the configuration files, with the `SERVICE_VARIANT` environment variable.

Cf https://github.com/edx/configuration/pull/234