Reuse test commands in cover target

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

Also install coverage only when needed.

Change-Id: I3bad06ca26524ec7125495fa38d1912930ef5ca9
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -1,6 +1,5 @@
 testtools>=1.4.0 # MIT
-coverage>=5.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 pifpaf>=0.10.0 # Apache-2.0
-stestr>=2.0.0
+stestr>=2.0.0 # Apache-2.0
 requests>=2.10.0 # Apache-2.0

tox.ini

@@ -33,8 +33,6 @@ allowlist_externals =
   {toxinidir}/run-tests.sh
 commands =
   {toxinidir}/run-tests.sh stestr run "{posargs}"
-passenv =
-  ETCD_VERSION
 
 [testenv:venv]
 # This target is used by the gate go run Sphinx to build the doc
@@ -48,11 +46,15 @@ commands =
   {toxinidir}/run-examples.sh
 
 [testenv:cover]
-setenv = {[testenv]setenv}
+setenv =
+  {[testenv]setenv}
   PYTHON=coverage run --source tooz --parallel-mode
+deps =
+  {[testenv]deps}
+  coverage>=5.0 # Apache-2.0
 commands =
   coverage erase
-  {toxinidir}/run-tests.sh stestr run "{posargs}"
+  {[testenv]commands}
   coverage combine
   coverage html -d cover
   coverage xml -o cover/coverage.xml