Allow use custom uppper constraints in stable/stein

Openstack Stein still supports Python 2.7 which means it is necessary to
set different upper constraints for that specific release for tempest
testing using that Python version.
The current master upper constraints won't work as they exclusively
consider Python version >= 3.x

Change-Id: Ieafd0ebc242eba2f4cbaf22823dbe94735236189
diff --git a/roles/run-tempest/README.rst b/roles/run-tempest/README.rst
index 3643edb..fa9599e 100644
--- a/roles/run-tempest/README.rst
+++ b/roles/run-tempest/README.rst
@@ -72,7 +72,7 @@
 .. zuul:rolevar:: stable_constraints_file
    :default: ''
 
-   Upper constraints file to be used for stable branch till stable/rocky.
+   Upper constraints file to be used for stable branch till stable/stein.
 
 .. zuul:rolevar:: tempest_tox_environment
    :default: ''
diff --git a/roles/run-tempest/tasks/main.yaml b/roles/run-tempest/tasks/main.yaml
index 1de3725..7f340e8 100644
--- a/roles/run-tempest/tasks/main.yaml
+++ b/roles/run-tempest/tasks/main.yaml
@@ -25,11 +25,11 @@
     target_branch: "{{ zuul.override_checkout }}"
   when: zuul.override_checkout is defined
 
-- name: Use stable branch upper-constraints till stable/rocky
+- name: Use stable branch upper-constraints till stable/stein
   set_fact:
     # TOX_CONSTRAINTS_FILE is new name, UPPER_CONSTRAINTS_FILE is old one, best to set both
     tempest_tox_environment: "{{ tempest_tox_environment | combine({'UPPER_CONSTRAINTS_FILE': stable_constraints_file}) | combine({'TOX_CONSTRAINTS_FILE': stable_constraints_file}) }}"
-  when: target_branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky"]
+  when: target_branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"]
 
 - name: Set OS_TEST_TIMEOUT if requested
   set_fact: