Update v3 identity project tag test to work w/ pre-prov

I don't see any limitations by using pre-provisioned
credentials for these test:

* test_list_update_delete_project_tags

By setting force_tenant_isolation=False these tests now be
can executed with backends that don't allow user creation
(immutable user source) like LDAP.

Change-Id: I04a0a00b8a33686038754f32c2f722e0e522fc4d
diff --git a/tempest/api/identity/admin/v3/test_project_tags.py b/tempest/api/identity/admin/v3/test_project_tags.py
index d05173b..b7878a8 100644
--- a/tempest/api/identity/admin/v3/test_project_tags.py
+++ b/tempest/api/identity/admin/v3/test_project_tags.py
@@ -25,6 +25,10 @@
 
 
 class IdentityV3ProjectTagsTest(base.BaseIdentityV3AdminTest):
+    # NOTE: force_tenant_isolation is true in the base class by default but
+    # overridden to false here to allow test execution for clouds using the
+    # pre-provisioned credentials provider.
+    force_tenant_isolation = False
 
     @decorators.idempotent_id('7c123aac-999d-416a-a0fb-84b915ab10de')
     @testtools.skipUnless(CONF.identity_feature_enabled.project_tags,