private List<RepositoryInfo> repoInfosForAllOs(String repoName, String repoVersion, String url) {
    List<RepositoryInfo> repos = new ArrayList<>(3);
    for (String os: new String[]{ "centos5", "centos6", "centos7"}) {
      repos.add(repoInfo(repoName, repoVersion, url, os));
    }
    return repos;
  }
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