Reuse test commands in cover target

... to maintain the commands in a single place.

Also install coverage only when needed.

Change-Id: Idd23e8fc7643ee49ec7adf80062b961aa17ee559
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,4 +1,3 @@
 oslotest>=3.2.0 # Apache-2.0
-coverage>=4.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 stestr>=2.0.0 # Apache-2.0

tox.ini

@@ -55,9 +55,12 @@ commands =
 [testenv:cover]
 setenv =
   PYTHON=coverage run --source oslo_concurrency --parallel-mode
+deps =
+  {[testenv]deps}
+  coverage>=5.0 # Apache-2.0
 commands =
   coverage erase
-  env TEST_EVENTLET=0 lockutils-wrapper stestr run {posargs}
+  {[testenv]commands}
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml