"property" as default source

[mlookaxw]

For the environmentalized fields a new property will be created in the configuration file. Default source for the property is "value".

The default source has to be changed to "property" to prevent missconfiguration.

Background:
For new attributes it is easy to forget to change the source attribute if the value attribute should contain a property name. If the default source is "property" the configuration process stops due to a missing property. This prevents from configuring a property source as value source by accident.