Reuse test commands in cover target

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

Also install coverage only when needed.

Change-Id: Ic6a13198bfb8be593d9fce077eedae2610bebc5c
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,4 +1,3 @@
-coverage>=5.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 # sphinx is needed for testing the sphinxext module
 sphinx>=2.0.0 # BSD

tox.ini

@@ -38,9 +38,12 @@ commands =
 [testenv:cover]
 setenv =
   PYTHON=coverage run --source stevedore --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