Remove deprecated allow_tenant_isolation option

allow_tenant_isolation option of auth and compute groups has been
deprecated over 3 years since I78a4884e980ef7d0103639cb3792a54c69fb7761.
So we have spended much enough time for removing them, this patch does.

Change-Id: I0b0a5e16a5e3785579f95f75fa149b311d918e8c
diff --git a/tempest/api/identity/v2/test_users.py b/tempest/api/identity/v2/test_users.py
index 9c77fff..158dfb3 100644
--- a/tempest/api/identity/v2/test_users.py
+++ b/tempest/api/identity/v2/test_users.py
@@ -84,7 +84,7 @@
         new_pass = data_utils.rand_password()
         user_id = self.creds.user_id
 
-        # to change password back. important for allow_tenant_isolation = false
+        # to change password back. important for use_dynamic_credentials=false
         self.addCleanup(self._restore_password, user_id, old_pass, new_pass)
 
         # user updates own password