_update-dns: fix error while looping on dns servers

If a DNS exception were raised during the loop, the dns_update message
were destroyed. On the next iteration of the loop, exception were raised
because of the NoneType instead of really trying next DNS server.

Thanks to Sebastien K. for the report and the debug !

acme_dns_tiny.py

@@ -49,8 +49,6 @@ def get_crt(config, log=LOGGER):
                 log.debug("Unable to %s DNS resource on server with IP %s, try again with "
                           "next available IP. Error detail: %s", action, nameserver, exception)
                 response = None
-            finally:
-                dns_update = None
             if response is not None:
                 break
         if response is None: