Cache the value of ca_is_enabled in the request context
    
    This may be called multiple times in a request and should
    be impossible to change.
    
    https://pagure.io/freeipa/issue/8797
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>