Reuse test commands in cover target

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

Also install coverage only when needed.

Change-Id: Ifa057f8ca22a462002519ac325deec209ddb84f6
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,4 +1,3 @@
-coverage>=5.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT

tox.ini

@@ -42,9 +42,12 @@ commands = {posargs}
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source automaton --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