ipv6 test failures: AttributeError: 'RRHeader' object has no attribute '_address'

[Björn Tillenius]

Without any changes to MAAS itself, tests have been started failing like this:

======================================================================
 FAIL: maasserver.tests.test_dhcp.TestGetDHCPConfigureFor.test_returns_for_ipv6
 maasserver.tests.test_dhcp.TestGetDHCPConfigureFor.test_returns_for_ipv6
 ----------------------------------------------------------------------
 testtools.testresult.real._StringException: testtools.testresult.real._StringException: Empty attachments:
   Twisted logs

 Seeds: {{{MAAS_RAND_SEED=27c83da3d809ac2a PYTHONHASHSEED=3680295492}}}

 Traceback (most recent call last):
   File "/run/build/checkout/src/provisioningserver/utils/network.py", line 780, in safe_getaddrinfo
     addr = IPAddress(hostname)
   File "/usr/lib/python3/dist-packages/netaddr/ip/__init__.py", line 305, in __init__
     raise AddrFormatError('failed to detect a valid IP ' \
 netaddr.core.AddrFormatError: failed to detect a valid IP address from 'localhost'

 During handling of the above exception, another exception occurred:

 Traceback (most recent call last):
   File "/run/build/checkout/src/maasserver/tests/test_dhcp.py", line 2510, in test_returns_for_ipv6
     ) = dhcp.get_dhcp_configure_for(
   File "/run/build/checkout/src/maasserver/dhcp.py", line 662, in get_dhcp_configure_for
     maas_dns_servers = get_default_dns_servers(
   File "/run/build/checkout/src/maasserver/dhcp.py", line 573, in get_default_dns_servers
     dns_servers = get_dns_server_addresses(
   File "/run/build/checkout/src/maasserver/dns/zonegenerator.py", line 153, in get_dns_server_addresses
     ips = get_maas_facing_server_addresses(
   File "/run/build/checkout/src/maasserver/server_address.py", line 76, in get_maas_facing_server_addresses
     addresses = resolve_hostname(
   File "/run/build/checkout/src/provisioningserver/utils/network.py", line 836, in resolve_hostname
     address_info = resolve_host_to_addrinfo(hostname, ip_version)
   File "/run/build/checkout/src/provisioningserver/utils/network.py", line 815, in resolve_host_to_addrinfo
     address_info = safe_getaddrinfo(
   File "/run/build/checkout/src/provisioningserver/utils/network.py", line 782, in safe_getaddrinfo
     result = _v6_lookup()
   File "/run/build/checkout/src/provisioningserver/utils/twisted.py", line 139, in wrapper
     return func_in_reactor(*args, **kwargs).wait(timeout)
   File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 198, in wait
     result.raiseException()
   File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 475, in raiseException
     raise self.value.with_traceback(self.tb)
 AttributeError: 'RRHeader' object has no attribute '_address'