Move 'gabbi_tempest_path' to gabbi-tempest plugin

'gabbi_tempest_path' var belong to gabbi-tempest and
as discussed in bug#1821072, we need to move this var
to its original owner.

Change-Id: I45f156ecd4617aa201c9457ee105b666b501dd03
Related-Bug: #1861592

.zuul.yaml

@@ -25,3 +25,4 @@
       devstack_localrc:
         TEMPEST_PLUGINS: "{{ devstack_base_dir|default('/opt/stack') }}/gabbi-tempest"
         USE_PYTHON3: True
+    run: playbooks/devstack-tempest-gabbi.yaml

playbooks/devstack-tempest-gabbi.yaml

@@ -0,0 +1,18 @@
+# Changes that run through devstack-tempest are likely to have an impact on
+# the devstack part of the job, so we keep devstack in the main play to
+# avoid zuul retrying on legitimate failures.
+- hosts: all
+  roles:
+    - orchestrate-devstack
+
+# We run tests only on one node, regardless how many nodes are in the system
+- hosts: tempest
+  environment:
+    # This enviroment variable is used by the optional tempest-gabbi
+    # job provided by the gabbi-tempest plugin.
+    GABBI_TEMPEST_PATH: "{{ gabbi_tempest_path | default('') }}"
+  roles:
+    - setup-tempest-run-dir
+    - setup-tempest-data-dir
+    - acl-devstack-files
+    - run-tempest