Pass tox_extra_args to Run tempest task

Patrole needs access to global packages, but the 'all-plugin' is
deprecated[0] and 'all' defines a sitepackages option as False[1].
Adding tox_extra_args option makes it possible to configure Zuul and
pass --sitepackages argument to tox.

[0] https://github.com/openstack/tempest/blob/9b4c50cf634cbea6e6a7353fff14c2f0a7ab1aa2/tox.ini#L64
[1] https://github.com/openstack/tempest/blob/9b4c50cf634cbea6e6a7353fff14c2f0a7ab1aa2/tox.ini#L7

Change-Id: Ib3264f54075216471720483c8dd816887c1f413c
diff --git a/roles/run-tempest/defaults/main.yaml b/roles/run-tempest/defaults/main.yaml
index c89eb93..06918b5 100644
--- a/roles/run-tempest/defaults/main.yaml
+++ b/roles/run-tempest/defaults/main.yaml
@@ -2,3 +2,4 @@
 tempest_test_regex: ''
 tox_envlist: smoke
 tempest_black_regex: ''
+tox_extra_args: ''