Disable logging to /srv by default

If an external storage is mounted, logging to /srv/log is enabled by default. As reported in the #29 (closed) issue, it may have unexpected and unwanted consequences (e.g. consumption of free space on the storage or slowing down due to eventual poor performance of the given storage). Thus I think that we should keep the logging disabled by default. It can be explicitly enabled if required.

Resolves #29 (closed).