RuleBase: Extract reading rules to a separate class

Logic to read rules from a configuration file is extracted to a separate
class. This has two benefits:

 * Simplify the RuleBase class and its corresponding test class
 * Prepare the way to introduce reading rules from project configuration
   which is done in a follow-up commit.

Change-Id: I6c0e1ef43da0f34d2ad0ba7001d205a492ecfe87