Simplify KRA transport cert cache
    
    In-memory cache causes problem in forking servers. A file based cache is
    good enough. It's easier to understand and avoids performance regression
    and synchronization issues when cert becomes out-of-date.
    
    https://pagure.io/freeipa/issue/6787
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>