ipatests: fix ipatests/test_xmlrpc/test_dns_plugin.py
    
    The test is calling dnsrecord-mod --ttl and should expect a unicode
    value in order to be python2/python3 compatible.
    
    Related: https://pagure.io/freeipa/issue/7982
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>