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

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
Merge pull request #136 from antoviaque/master

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