Always enable project tags Identity tests

The feature is enabled since Queens, so turn on the testing by default
and deprecate the option.

Change-Id: I72eb8f9640a45a069582dfb018ce7317d4a8c32e

releasenotes/notes/deprecate-and-enable-identity-project-tags-23b87518888e563a.yaml

@@ -0,0 +1,17 @@
+---
+upgrade:
+  - |
+    Project tags are supported by Keystone since Queens.
+    As Tempest currently supports only much newer OpenStack versions
+    (Ussuri or later), the configuration option which enables
+    project tags testing
+    (``CONF.identity-feature-enabled.project_tags``)
+    is now enabled by default.
+deprecations:
+  - |
+    Project tags are supported by Keystone since Queens.
+    As Tempest currently supports only much newer OpenStack versions
+    (Ussuri or later), the configuration option which enables
+    project tags testing
+    (``CONF.identity-feature-enabled.project_tags``)
+    is now deprecated.

tempest/config.py

@@ -259,8 +259,11 @@ IdentityFeatureGroup = [
                 help='Does the environment have the security compliance '
                      'settings enabled?'),
     cfg.BoolOpt('project_tags',
-                default=False,
-                help='Is the project tags identity v3 API available?'),
+                default=True,
+                help='Is the project tags identity v3 API available?',
+                deprecated_for_removal=True,
+                deprecated_reason='Project tags API is a default feature '
+                                  'since Queens'),
     cfg.BoolOpt('application_credentials',
                 default=True,
                 help='Does the environment have application credentials '