aggregation plugin: Implement the "GroupBy" option.

aggregation plugin: Implement the "GroupBy" option.

This new configuration format has two benefits:
1) It is easier to understand by users, because they don't have to know
   about two types of wildcards.
2) In the future matching of Host, Plugin, Type, ... may support regular
   expressions. In that case, this configuration syntax doesn't need to
   be adapted.