py3: write CA/KRA config into file opened in text mode
    
    config parser writes data as text so CA/KRA should be opened in textual
    mode otherwise type errors are raised from installer
    
    https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>