@Override
  public void verifyRepositories(Set<RepositoryRequest> requests) throws AmbariException {
    for (RepositoryRequest request: requests) {
      if (request.getBaseUrl() == null) {
        throw new AmbariException("Base url is missing for request " + request);
      }
      verifyRepository(request);
    }
  }
Chooses a healthy host from the list of candidate hosts randomly. If there
are no healthy hosts, then this method will return {@code null}.
<p/>
The host's {@link HostState} is used to determine if a host is healthy.

@return a random healthy host, or {@code null}.
@throws AmbariException
@see {@link HostState#HEALTHY}
ambari-server/src/main/java/org/apache/ambari/server/controller/AmbariManagementControllerImpl.java