Travis: Add workaround for missing IPv6 support
    
    Latest Travis CI image lacks IPv6 address on localhost. Add some
    diagnostics and skip IPv6 tests in ipa-server-install when TRAVIS is
    detected.
    
    The hack will be removed as soon as it is no longer required to pass
    automated testing.
    
    https://pagure.io/freeipa/issue/7323
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Felipe Volpone <fbarreto@redhat.com>