Consolidate all services to use the Reva config struct for the shared package.
This works because all services (except 'notifications', 'thumbnails' and
'webdav') where using the same config keys and environment variables for
setting the reva gateway.
Up to now the builtin lico was using the "username" as the login
attribute, while the proxy (and to some extend the auth-basic) service
tried to uniquely identify users by mail address. This aligns the
default configuration of the services to use the username everywhere.
Fixes: #4039
The help flag is configured automatically by default already. We don't
need to redo that for every single service.
This also addresses one of the finding of "go race" (#4088)
