Improve `cover` tox env
Improve `cover` tox env

Reuse `setenv` settings from parent `testenv` env

This is a followup to I3aefd97312bb9e5985e601dd192913247046d6e4

Change-Id: I4960a737a68ee146447cb6e4a5c6fe5b36a6dfc1

openstack/ironic-staging-drivers,openstack/ironic-staging-drivers