Configuration resolution

[vadeg]

The issue has been identified in #1289 (See this comment for details).

Zally rule sets use lightbend config and reference.conf files define configuration. These files as supposed to be used for default configuration values. When library reads configuration, it merges all properties from reference.conf files into one configuration. The merging order is undefined.

The problem appears when a developer wants to develop a new rule set which has overlapping configuration with one of existing rule sets. In this case the final value of overlapping configuration property is unknown. Developers can override property in the main configuration loaded by the library. In case of Zally it is rules-config.yaml in the zally-server module. This means that rule set specific configuration should be defined in zally-server (or another main configuration) which is not the right place.

The goal of this ticket to explore how the issue with overlapping configurations can be solved.