ipatests: fix host name for ssh connection from controller to master
    
    Use master.external_hostname instead of master.hostname for ssh connection
    from controller machine to master. If hostname and external_hostname in
    test_config.yml do no match then trying to establish ssh connection
    was failing with "[Errno -2] Name or service not known".
    
    Fixes https://pagure.io/freeipa/issue/7874