[CI] test_create_dynamic_range sometimes fails in CI

Every so often we're seeing test_create_dynamic_range fail with:

testtools.testresult.real._StringException: Empty attachments:
  stderr for maas maas ['ipranges', 'create', 'type=dynamic', 'start_ip=10.245.136.10', 'end_ip=10.245.136.200']

retcode for maas maas ['ipranges', 'create', 'type=dynamic', 'start_ip=10.245.136.10', 'end_ip=10.245.136.200']: {{{2}}}
stdout for maas maas ['ipranges', 'create', 'type=dynamic', 'start_ip=10.245.136.10', 'end_ip=10.245.136.200']: {{{{"subnet": ["This field is required."]}}}}

Traceback (most recent call last):
  File "/tmp/autopkgtest.9UO1rq/build.KY6/real-tree/debian/tests/maas-integration.py", line 604, in test_create_dynamic_range
    self.assertThat(output, Contains('"cidr": "%s"' % REGION_DHCP_CONFIG['cidr']))
  File "/usr/lib/python3/dist-packages/testtools/testcase.py", line 435, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: '"cidr": "10.245.136.0/21"' not in '{"subnet": ["This field is required."]}'

It's only occurring every dozen builds or so, so it's not a huge pain at the moment, but we should address it if we can.