Support node-specific configuration

This configuration should be stored as org.appng.core.domain.PropertyImpl instances and made available as org.appng.api.model.Properties.
These Properties should be put into the PLATFORM-Scoped org.appng.api.Environment with the key nodeConfig (create a constant in org.appng.api.Platform.Environment).

Therefore, a new property namespace needs to be created.
My proposal is platform.node.<node-identifier>.<property_name>.

For retrieving the <node-identifier>, org.appng.api.messaging.Messaging#getNodeId(Environment env) should be used, but since property names are separated by a dot, these must be replaced with an underscore.

An example usage scenario:
Each node can be tagged/labeled with several keywords.
These keyword can then also get assigned to the sites.
When starting the appNG Node, only sites with the same tags/labels as the current node are getting loaded when appNG starts.