Skip test_create_update_server_with_subnet

Let's skip test_create_update_server_with_subnet from
heat_tempest_plugin because it is constantly failing on a timeout
issue:
'failed to reach UPDATE_COMPLETE status within the required time
(1200 s)'
The test is working locally so the failure is not caused by any
tempestconf's recent change or by a tempest.conf issue in general.
In order to unblock tempestconf's CI, the patch excludes the test
in python-tempestconf-tempest-devstack-admin-plugins job.

Change-Id: If9f9b3be758897f25d16326aa6d0f7c53e110122

.zuul.yaml

@@ -173,6 +173,7 @@
         - 'designate_tempest_plugin'
         - 'barbican_tempest_plugin'
       black_regex:
+        - 'heat_tempest_plugin.tests.functional.test_nova_server_networks.CreateServerTest.test_create_update_server_with_subnet'
         - 'heat_tempest_plugin.tests.scenario.test_volumes.VolumeBackupRestoreIntegrationTest.test_cinder_volume_create_backup_restore'
         - 'heat_tempest_plugin.tests.scenario.test_server_software_config.SoftwareConfigIntegrationTest.test_server_software_config'
         - 'heat_tempest_plugin.tests.scenario.test_server_signal.ServerSignalIntegrationTest.test_server_signal_userdata_format_software_config'