Fix: do not lose the tox_environment value in func tests

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

playbooks/tox/run-both.yaml

@@ -8,5 +8,4 @@
     - test-setup
     - ensure-tox
     - get-devstack-os-environment
-    - role: tox
-      tox_environment: "{{ os_env_vars|default({}) }}"
+    - tox

playbooks/tox/run.yaml

@@ -1,5 +1,4 @@
 - hosts: all
   roles:
     - get-devstack-os-environment
-    - role: tox
-      tox_environment: "{{ os_env_vars|default({}) }}"
+    - tox

roles/get-devstack-os-environment/defaults/main.yaml

@@ -3,3 +3,4 @@ openrc_file: "{{ devstack_base_dir }}/devstack/openrc"
 openrc_user: admin
 openrc_project: admin
 openrc_enable_export: false
+tox_environment: {}

roles/get-devstack-os-environment/tasks/main.yaml

@@ -9,6 +9,6 @@
         executable: "/bin/bash"
       register: env_os
 
-    - name: Save the OS_ environment variables as a fact
+    - name: Append the the OS_ environment variables to tox_environment
       set_fact:
-        os_env_vars: "{{ env_os.stdout|from_yaml }}"
+        tox_environment: "{{ env_os.stdout|from_yaml|default({})|combine(tox_environment) }}"