Add 'host_type', make it possible to use Windows hosts
    
    Add global parameter windows_test_dir to specify test directory
    for Windows
    Windows hosts (WinHost) use this directory by default.
    test_dir can now be set individually for each host.
    Move run_command from Host class to BaseHost class
    Add a "host_type" configuration option and Host attribute. This
    is used to select the Host subclass. By default it can
    be 'default' or 'windows' (but Config subclasses can define more).
    
    Signed-off-by: Petr Viktorin <pviktroi@redhat.com>