Reuse test commands in cover target

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

Also install coverage only when needed.

Change-Id: I91298ba5a5d5221a9bcaf3bfe6cf1dab9f92c36d
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,7 +1,6 @@
 mypy>=1.19.0 # MIT
 oslotest>=3.2.0 # Apache-2.0
 testtools>=2.2.0 # MIT
-coverage>=5.0 # Apache-2.0
 jsonschema>=3.2.0 # MIT
 stestr>=2.0.0 # Apache-2.0
 

tox.ini

@@ -35,9 +35,12 @@ commands = {posargs}
 [testenv:cover]
 setenv =
   PYTHON=coverage run --source oslo_versionedobjects --parallel-mode
+deps =
+  {[testenv]deps}
+  coverage>=5.0 # Apache-2.0
 commands =
   coverage erase
-  stestr run {posargs}
+  {[testenv]commands}
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml