Reuse test commands in cover target

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

Also install coverage only when needed.

Change-Id: Ibf4b1e7eae3ad780ce28ab481d9fd8365b0090f3
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,4 +1,3 @@
-coverage>=5.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT

tox.ini

@@ -29,9 +29,12 @@ commands =
 [testenv:cover]
 setenv =
   PYTHON=coverage run --source pycadf --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