Fix passenv format unaccepted by tox 4

tox 4 does not accept updated its INI rules and the current format
violates the new rules.

https: //github.com/tox-dev/tox/blob/main/docs/upgrading.rst#changed-ini-rules
Change-Id: I9eb9d03d33661ed2bfc6ce5e57dba9656483c51e

.zuul.yaml

@@ -12,6 +12,10 @@
         - vitrage-tempest-plugin-api-ipv6-only
         - vitrage-tempest-plugin-datasources
         - vitrage-grenade
+        - openstack-tox-py38:
+            voting: false
+        - openstack-tox-py310:
+            voting: false
 
     gate:
       jobs:
@@ -19,6 +23,10 @@
         - vitrage-tempest-plugin-api-ipv6-only
         - vitrage-tempest-plugin-datasources
         - vitrage-grenade
+        - openstack-tox-py38:
+            voting: false
+        - openstack-tox-py310:
+            voting: false
 
 - job:
     name: vitrage-grenade

tox.ini

@@ -19,7 +19,13 @@ commands =
     stestr slowest
     oslo-config-generator --config-file=etc/vitrage/vitrage-config-generator.conf
     find . -type f -name "test-*.db" -delete
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+    http_proxy
+    HTTP_PROXY
+    https_proxy
+    HTTPS_PROXY
+    no_proxy
+    NO_PROXY
 
 [testenv:pep8]
 commands = flake8