batch: add keeponly option
    
    batch(methods=Dict(), keeponly=list) will allow to execute batch of
    commands and remove from the output everything but the attributes which
    names were passed in the keeponly list.
    
    This can be useful if you are only interested in getting names and
    assigned random passwords, for example.
    
    Fix batch API test in test_integration/test_idm_api.py and use it to
    validate keeponly option.
    
    Fixes: https://pagure.io/freeipa/issue/9583
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Thomas Woerner <twoerner@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Rafael Guterres Jeffman <rjeffman@redhat.com>