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
diff --git a/tempest/config.py b/tempest/config.py
index fde52e4..662a249 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -259,8 +259,11 @@
                 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 '