private RepositoryInfo repoInfo(String repoName, String repoVersion, String url) {
    return repoInfo(repoName, repoVersion, url, "centos6");
  }
If two add-on services define the same repo, the repo should be disregarded.
This applies per os, so the same repo can be defined for multiple os'es (e.g redhat5 and redhat6)
@throws Exception
ambari-server/src/test/java/org/apache/ambari/server/stack/StackModuleTest.java