Make hidden stack tag configurable

the current hardcoded value is different from the default one in Heat
and is mainly applicable to a Heat deployed with devstack-gate on CI.

To make this test pass against an OpenStack cloud in a general case
the tag that must be considered by the test to be hidden must be
configurable.

Change-Id: I51e03ee919536b5946271f2f9abc78e382dd00cb
Closes-Bug: #1757117
diff --git a/heat_tempest_plugin/config.py b/heat_tempest_plugin/config.py
index d658a98..ced87e3 100644
--- a/heat_tempest_plugin/config.py
+++ b/heat_tempest_plugin/config.py
@@ -153,7 +153,9 @@
     cfg.StrOpt('heat_config_notify_script',
                default=('heat-config-notify'),
                help="Path to the script heat-config-notify"),
-
+    cfg.StrOpt('hidden_stack_tag',
+               default='data-processing-cluster',
+               help="Tag to be considered as hidden for stack tags tests"),
 ]
 
 
diff --git a/heat_tempest_plugin/tests/functional/test_stack_tags.py b/heat_tempest_plugin/tests/functional/test_stack_tags.py
index 7d3d52a..55c262d 100644
--- a/heat_tempest_plugin/tests/functional/test_stack_tags.py
+++ b/heat_tempest_plugin/tests/functional/test_stack_tags.py
@@ -72,7 +72,7 @@
     @decorators.idempotent_id('5ed79584-0684-4f9c-ae8e-44a8f874ec79')
     def test_hidden_stack(self):
         # Stack create with hidden stack tag
-        tags = 'foo,hidden'
+        tags = 'foo,%s' % self.conf.hidden_stack_tag
         self.stack_create(
             template=self.template,
             tags=tags)