Use stable constraint in run-tempest for stable branch testing

Stable branches till stable/rocky is using python
version <py3.6. Tempest test those branch in venv
but Tempest tox use the master upper-constraint[1]
which block installation due to dependencies
require >=py3.6. For exmaple, oslo.concurrency 4.0.0
is not compatible for <py3.6.

As we pin Tempest for EM stable brach, we should be
able to use stable constraint for Tempest installation
as well as while running during run-tempest playbook.

tox.ini is hard coded to use master constraint[1] which force
run-tempest to recreate the tox env and use the master constraint.

Devstack can set stable u-c to use via env var but this is
not enough as Tempest role run-tempest recreate the tox
env with master u-c.
- https://review.opendev.org/#/c/705089/4/lib/tempest

Currently we are fixing run-tempest to use stable constraint
till stable/rocky which use distro having <py3.6.

Ideally, we can change compatible branch constraint in tox.ini
while doing release tag for Tempest. But that can be done
for future tags only.

[1] bc9fe8eca8/tox.ini (L14)

Change-Id: I4268049ff91e5527e4b95fd8a70fa53ba27d56ed

roles/run-tempest/README.rst

@@ -72,3 +72,16 @@ Run Tempest
    :default: ''
 
    The timeout (in seconds) for each test.
+
+.. zuul:rolevar:: stable_constraints_file
+   :default: ''
+
+   Upper constraints file to be used for stable branch till stable/rocky.
+
+.. zuul:rolevar:: tempest_tox_environment
+   :default: ''
+
+   Environment variable to set for run-tempst task.
+
+   Env variables set in this variable will be combined with some more
+   defaults env variable set at runtime.

roles/run-tempest/defaults/main.yaml

@@ -4,3 +4,6 @@ tox_envlist: smoke
 tempest_black_regex: ''
 tox_extra_args: ''
 tempest_test_timeout: ''
+stable_constraints_file: "{{ devstack_base_dir }}/requirements/upper-constraints.txt"
+target_branch: "{{ zuul.branch }}"
+tempest_tox_environment: {}

roles/run-tempest/tasks/main.yaml

@@ -20,6 +20,21 @@
     default_concurrency: "{{ num_cores|int // 2 }}"
   when: num_cores|int > 3
 
+- name: Override target branch
+  set_fact:
+    target_branch: "{{ zuul.override_checkout }}"
+  when: zuul.override_checkout is defined
+
+- name: Use stable branch upper-constraints till stable/rocky
+  set_fact:
+    tempest_tox_environment: "{{ tempest_tox_environment | combine({'UPPER_CONSTRAINTS_FILE': stable_constraints_file}) }}"
+  when: target_branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky"]
+
+- name: Set OS_TEST_TIMEOUT if requested
+  set_fact:
+    tempest_tox_environment: "{{ tempest_tox_environment | combine({'OS_TEST_TIMEOUT': tempest_test_timeout}) }}"
+  when: tempest_test_timeout != ''
+
 - when:
     - tempest_test_blacklist is defined
   block:
@@ -42,4 +57,4 @@
     chdir: "{{devstack_base_dir}}/tempest"
   become: true
   become_user: tempest
-  environment: '{{ {"OS_TEST_TIMEOUT": tempest_test_timeout} if tempest_test_timeout else {} }}'
+  environment: "{{ tempest_tox_environment }}"