IXFR on SECONDARY zone fails

[Andy Gomez]

When records are added to the master ( bind9 ) designate-mdns fails receiving the IXFR from the master.

AXFR's do work however.

designate-mdns[8717]: Traceback (most recent call last):
designate-mdns[8717]: File "/usr/lib/python2.7/dist-packages/eventlet/hubs/hub.py", line 457, in fire_timers
designate-mdns[8717]: timer()
designate-mdns[8717]: File "/usr/lib/python2.7/dist-packages/eventlet/hubs/timer.py", line 58, in __call__
designate-mdns[8717]: cb(*args, **kw)
designate-mdns[8717]: File "/usr/lib/python2.7/dist-packages/eventlet/greenthread.py", line 214, in main
designate-mdns[8717]: result = function(*args, **kwargs)
designate-mdns[8717]: File "/usr/lib/python2.7/dist-packages/designate/mdns/xfr.py", line 36, in zone_sync
designate-mdns[8717]: servers = servers.to_list()
designate-mdns[8717]: AttributeError: 'list' object has no attribute 'to_list'

 openstack zone show 4e52a80f-7407-40a1-b2c2-e1301c5503b4
+----------------+--------------------------------------+
| Field | Value |
+----------------+--------------------------------------+
| action | NONE |
| attributes | |
| created_at | 2020-05-19T21:39:20.000000 |
| description | None |
| email | <email address hidden> |
| id | 4e52a80f-7407-40a1-b2c2-e1301c5503b4 |
| masters | 10.10.10.11 |
| name | example.com. |
| pool_id | 794ccc2c-d751-44fe-b57f-8894c9f5c842 |
| project_id | 8ab1df6f17dd49ceb9ef0f1e9b6f2bf5 |
| serial | 2020042242 |
| status | ACTIVE |
| transferred_at | 2020-05-20T16:22:15.000000 |
| ttl | 604800 |
| type | SECONDARY |
| updated_at | 2020-05-20T16:23:16.000000 |
| version | 6 |
+----------------+--------------------------------------+

apt-cache policy designate
designate:
  Installed: 1:6.0.1-0ubuntu1.2~cloud0
  Candidate: 1:6.0.1-0ubuntu1.2~cloud0