Install coverage only when needed

It's required only when generating a coverage report.

Change-Id: I09ba4c7ddeb9751a1fec95de6daf599cac301919
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

test-requirements.txt

@@ -9,6 +9,4 @@ oslotest>=3.2.0 # Apache-2.0
 # test environment.
 oslo.log>=3.36.0  # Apache-2.0
 
-coverage>=4.0 # Apache-2.0
-
 requests-mock>=1.5.0 # Apache-2.0

tox.ini

@@ -43,6 +43,9 @@ commands =
 [testenv:cover]
 setenv =
   PYTHON=coverage run --source oslo_config --parallel-mode
+deps =
+  {[testenv]deps}
+  coverage>=5.0 # Apache-2.0
 commands =
   coverage erase
   {[testenv]commands}