Reduce stestr concurrency to 1 for tempest jobs

The switch from ostestr to stestr introduced in Change-Id
Ie6a275170ec6faf0db3976410660f52078710a54 missed setting concurrency
jobs to 1 for api and scenario tests, leading to intermittent gate
issues.

This patch also fixes coverage test that got broken in the
aforementioned Change-Id.

Depends-On: https://review.openstack.org/#/c/542223/
Change-Id: I077330ad6f0308845d1b5330040be6f92c39580d
(cherry picked from commit d871c2a7fb)

.testr.conf

@@ -1,5 +0,0 @@
-[DEFAULT]
-#test_command=${PYTHON:-python} -m subunit.run discover octavia $LISTOPT $IDOPTION
-test_command=OS_STDOUT_CAPTURE=1 OS_STDERR_CAPTURE=1 OS_LOG_CAPTURE=1 ${PYTHON:-python} -m subunit.run discover -t ./ ${OS_TEST_PATH:-./octavia/tests/unit} $LISTOPT $IDOPTION
-test_id_option=--load-list $IDFILE
-test_list_option=--list

tox.ini

@@ -30,11 +30,14 @@ commands =
 
 [testenv:cover]
 whitelist_externals = sh
+setenv =
+    {[testenv]setenv}
+    PYTHON=coverage run --source octavia --parallel-mode
 commands =
     coverage erase
-    sh -c 'COVERAGE_FILE={toxinidir}/.coverage-py27 python setup.py testr --coverage --testr-args='{posargs}''
-    sh -c 'COVERAGE_FILE={toxinidir}/.coverage-functional OS_TEST_PATH={toxinidir}/octavia/tests/functional python setup.py testr --coverage --testr-args='{posargs}''
-    coverage combine {toxinidir}/.coverage-py27 {toxinidir}/.coverage-functional
+    sh -c 'OS_TEST_PATH={toxinidir}/octavia/tests/unit stestr run {posargs}'
+    sh -c 'OS_TEST_PATH={toxinidir}/octavia/tests/functional stestr run {posargs}'
+    coverage combine
     coverage report --fail-under=80 --skip-covered
     # Generate a new HTML report with the combined results
     # otherwise the HTML report will only show partial results
@@ -151,7 +154,9 @@ commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasen
 passenv = TEMPEST_CONFIG_DIR
 setenv =
  OS_TEST_PATH={toxinidir}/octavia/tests/tempest/v1/api
- OS_TESTR_CONCURRENCY=1
+commands =
+  stestr run --concurrency 1 '{posargs}'
+  stestr slowest
 
 # If you are running the tests locally you should set the env variable
 # TEMPEST_CONFIG_DIR=/opt/stack/tempest/etc
@@ -159,4 +164,6 @@ setenv =
 passenv = TEMPEST_CONFIG_DIR
 setenv =
  OS_TEST_PATH={toxinidir}/octavia/tests/tempest/v1/scenario
- OS_TESTR_CONCURRENCY=1
+commands =
+  stestr run --concurrency 1 '{posargs}'
+  stestr slowest