Use queue for get_subdomains
    
    It does not make much sense to run multiple get_subdomains request in
    parallel because all requests will load the same information from the
    server. The IPA and AD provider already implement a short timeout to
    avoid the multiple requests are running to fast after each other. But if
    the timeout is over chances are that if two or more request come in fast
    the first request cannot update the timeout and request will run in
    parallel. To avoid this the requests are queued and send one after the
    other to the provider.