py3: ipaldap: properly encode DNSName to bytes
The encode method of LDAPClient didn't return DNSName as bytes but
string in py3. In py2 it returns non-unicode string so it can be encoded
safely by ascii as to_text() method returns only ascii characters.
https://fedorahosted.org/freeipa/ticket/4985
Reviewed-By: Jan Cholasta <jcholast@redhat.com>