cert-find: fix 'issuer' option
    
    The 'issuer' option of cert-find was recently changed from Str to
    DNParam, however, 'ra.find' expects a string and throws when it
    receives a DN.
    
    When constructing the dict that gets passed to 'ra.find', turn
    DNParams into strings.
    
    Part of: https://fedorahosted.org/freeipa/ticket/5381
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>