The current code always overrides tox_environment when running
functional tests, but the correct behavior is to add
the discovered environment variables to tox_environments,
while keeping the user-specified value for it.
The current behavior breaks the devstack-tox-functional children
jobs, like openstacksdk-functional-devstack-ironic, which set
tox_environment.
Change-Id: I5dc9054a1495ca0ef7745c08316441ab153956f4
906d824a19