zuul: new variable to easily populate TEMPEST_PLUGINS

TEMPEST_PLUGINS contains the list of the tempest plugins installed
alongside tempest by lib/tempest.
If TEMPEST_PLUGINS is not explicitly set, the new tempest_plugins
variable is used to fill it by combining its items with
the base devstack path.

Change-Id: I9f1fa2755e16871ff9d6ba33fdeaf3023eedf8d4
diff --git a/roles/write-devstack-local-conf/tasks/main.yaml b/roles/write-devstack-local-conf/tasks/main.yaml
index 9a6b083..bfd0860 100644
--- a/roles/write-devstack-local-conf/tasks/main.yaml
+++ b/roles/write-devstack-local-conf/tasks/main.yaml
@@ -10,4 +10,5 @@
     local_conf: "{{ devstack_local_conf|default(omit) }}"
     base_dir: "{{ devstack_base_dir|default(omit) }}"
     projects: "{{ zuul.projects }}"
-    project: "{{ zuul.project }}"
\ No newline at end of file
+    project: "{{ zuul.project }}"
+    tempest_plugins: "{{ tempest_plugins|default(omit) }}"