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] 9b4c50cf63/tox.ini (L64)
[1] 9b4c50cf63/tox.ini (L7)

Change-Id: Ib3264f54075216471720483c8dd816887c1f413c

roles/run-tempest/README.rst

@@ -56,3 +56,14 @@ Run Tempest
                (?x)    # Ignore comments and whitespaces
                # Line with only a comment.
                (tempest.api.identity).*$
+
+.. zuul:rolevar:: tox_extra_args
+   :default: ''
+
+   String of extra command line options to pass to tox.
+
+   Here is an example of running tox with --sitepackages option:
+
+       ::
+           vars:
+             tox_extra_args: --sitepackages

roles/run-tempest/defaults/main.yaml

@@ -2,3 +2,4 @@ devstack_base_dir: /opt/stack
 tempest_test_regex: ''
 tox_envlist: smoke
 tempest_black_regex: ''
+tox_extra_args: ''

roles/run-tempest/tasks/main.yaml

@@ -35,7 +35,7 @@
       when: blacklist_stat.stat.exists
 
 - name: Run Tempest
-  command: tox -e {{tox_envlist}} -- {{tempest_test_regex|quote}} {{blacklist_option|default('')}} \
+  command: tox -e {{tox_envlist}} {{tox_extra_args}} -- {{tempest_test_regex|quote}} {{blacklist_option|default('')}} \
             --concurrency={{tempest_concurrency|default(default_concurrency)}} \
             --black-regex={{tempest_black_regex|quote}}
   args: