Migration warning when compat enabled
    
    Added check into migration plugin to warn user when compat is enabled.
    If compat is enabled, the migration fails and user is warned that he
    must turn the compat off or run the script with (the newly introduced)
    option '--with-compat'.
    
    '--with-compat' is new flag. If it is set, the compat status is ignored.
    
    https://fedorahosted.org/freeipa/ticket/2274