Refactor waiters in our tempest plugin

Deduplicates some of the code between common.waiters module and
BaremetalScenarioTest functions.

Change-Id: Ia87646cccdefba22caf121f4e347e8f2edf736e0
diff --git a/ironic_tempest_plugin/common/utils.py b/ironic_tempest_plugin/common/utils.py
new file mode 100644
index 0000000..67c4922
--- /dev/null
+++ b/ironic_tempest_plugin/common/utils.py
@@ -0,0 +1,33 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+
+def get_node(client, node_id=None, instance_uuid=None):
+    """Get a node by its identifier or instance UUID.
+
+    If both node_id and instance_uuid specified, node_id will be used.
+
+    :param client: an instance of tempest plugin BaremetalClient.
+    :param node_id: identifier (UUID or name) of the node.
+    :param instance_uuid: UUID of the instance.
+    :returns: the requested node.
+    :raises: AssertionError, if neither node_id nor instance_uuid was provided
+    """
+    assert node_id or instance_uuid, ('Either node or instance identifier '
+                                      'has to be provided.')
+    if node_id:
+        _, body = client.show_node(node_id)
+        return body
+    elif instance_uuid:
+        _, body = client.show_node_by_instance_uuid(instance_uuid)
+        if body['nodes']:
+            return body['nodes'][0]