Missing default value for loglevel

While working on a docs PR where a minimum loglevel is a requirement, I saw that:

* we offer on each service a loglevel, but we do not have a [default value](https://doc.owncloud.com/ocis/next/deployment/services/s-list/idm.html#environment-variables) defined,
though it may be implicit. We should make the default explicit.
* the loglevel `disabled`, though offered by the package, is not listed as an available loglevel.
Such a loglevel could be important for particular services where you want exclude logging sensitive data while others go into details with a debug loglevel. (global vs service loglevel)

Discussed with @dragonchaser

@micbar how to proceed?