Race condition in zone imports tests

Seen here: http://logs.openstack.org/23/523223/3/check/puppet-openstack-integration-4-scenario003-tempest-centos-7/212f9ac/job-output.txt.gz#_2017-11-28_09_29_27_411291

designate_tempest_plugin.tests.api.v2.test_zones_imports.ZonesImportTest.test_show_zone_import[id-c8909558-0dc6-478a-9e91-eb97b52e59e0,smoke]
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Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
      File "/usr/lib/python2.7/site-packages/designate_tempest_plugin/tests/api/v2/test_zones_imports.py", line 55, in test_show_zone_import
        self.assertExpected(zone_import, body, self.excluded_keys)
      File "/usr/lib/python2.7/site-packages/designate_tempest_plugin/tests/base.py", line 82, in assertExpected
        self.assertEqual(value, actual[key], key)
      File "/usr/lib/python2.7/site-packages/testtools/testcase.py", line 350, in assertEqual
        self.assertThat(observed, matcher, message)
      File "/usr/lib/python2.7/site-packages/testtools/testcase.py", line 435, in assertThat
        raise mismatch_error
    testtools.matchers._impl.MismatchError: None != u'3e9ab13d-5b17-47f9-b165-c3e84c8439a9': zone_id

We can see that returned by the import POST:

Body: {"status": "PENDING", "zone_id": null, ...}

So the zone_id is not available yet, we need to poll to make sure the zone is created properly before calling show on it.