New API test - test_list_service_statuses

Test scenario:
  Initiate "Service Statuses" API request and make sure that:
  1) All mandatory services are listed in API responce
  2) Each listed service is in "UP" status

Change-Id: Ib11297d5e17df8caf9e8b533c3dccdbe97899dec
diff --git a/designate_tempest_plugin/common/constants.py b/designate_tempest_plugin/common/constants.py
new file mode 100644
index 0000000..7ebcc87
--- /dev/null
+++ b/designate_tempest_plugin/common/constants.py
@@ -0,0 +1,16 @@
+# Copyright 2021 Red Hat.
+#
+# 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.
+
+# API statuses
+UP = 'UP'