Make o.a.l.l.c.config.BaseConfiguration abstract.

  - Removed unused field.
  - Using this as an abstract class to help refactor these 
  configuration classes.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1580614 13f79535-47bb-0310-9956-ffa450edef68