Reuse test commands in cover target

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

Also install coverage only when needed.

Change-Id: I467ccc1485d04edbeb70944b871c4f477e719169
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,3 +1,2 @@
 stestr>=3.0.1 # Apache-2.0
-coverage>=5.0 # Apache-2.0
 oslotest>=1.10.0 # Apache-2.0

tox.ini

@@ -29,9 +29,12 @@ commands = {posargs}
 [testenv:cover]
 setenv =
   PYTHON=coverage run --source cursive --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