Implemented resolveNodeKey() in AbstractHierarchicalConfiguration.

Some refactoring to remove duplicated code.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580038 13f79535-47bb-0310-9956-ffa450edef68
Implemented nodeKey() in AbstractHierarchicalConfiguration.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580039 13f79535-47bb-0310-9956-ffa450edef68