extracted mergeConfiguration method

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1465274 13f79535-47bb-0310-9956-ffa450edef68
[MSHARED-271] support multiple execution of a report because it can happen in multiple reportSets

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1465279 13f79535-47bb-0310-9956-ffa450edef68