Simplify configuration (pt. 2)

[bertfrees]

See also part 1.

• Split properties into "user properties" and "internal properties". The internal ones should be removed from the documentation and it should not be possible to set them with environment variables (nor with Maven properties at build time). Possibly add a section "only if you know what you are doing" to the documentation, with properties such as "updateSite" in it.
• Centralize definition of default values (currently in system.properties, start script, Java code, /etc/default, ...)
• Convert Maven properties to default Java system properties at build time