Add openstack cover job

The patch adds a new job which runs cover tox environment
to show code coverage by unit tests.

Change-Id: I888cd729b9a1dbe65a3380da357eafc1cc961b36

.zuul.yaml

@@ -1,5 +1,6 @@
 - project:
     templates:
+      - openstack-cover-jobs
       - openstack-python37-jobs
       - openstack-python3-wallaby-jobs
     check:

tox.ini

@@ -16,6 +16,7 @@ commands =
 
 whitelist_externals =
     bash
+    find
 distribute = false
 
 [testenv:pep8]
@@ -29,10 +30,13 @@ commands = {posargs}
 
 [testenv:cover]
 basepython = python3
+setenv =
+  {[testenv]setenv}
+  PYTHON=coverage run --source refstack_client --parallel-mode
 commands =
   coverage erase
   find . -type f -name "*.pyc" -delete
-  stestr --test-path ./refstack_client/tests run {posargs}
+  stestr run {posargs}
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml