Spurious failures in DNS configuration tests

[Gavin Panella]

There is some underlying issue in the DNS configuration tests in maasserver.dns.tests.test_config. I have already filed bug 1546475, bug 1549206, and bug 1549208 for individual failures. A recent branch landing [1] uncovered 3 more: test_edit_subnet_updates_DNS_zone, test_dns_update_zones_now_changes_dns_zone, test_bind_configuration_includes_ipv6_zone. Most of these tests are in TestDNSConfigModifications but the last was in TestIPv6DNS, so I'm taking the rather drastic step of disabling ALL tests in that module to unblock landing branches to trunk.

[1] https://code.launchpad.net/~allenap/maas/flaky-subnet-test/+merge/286924/comments/732057, but see the whole landing history of that merge proposal.