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/README.rst b/roles/write-devstack-local-conf/README.rst
index e9739cd..d0a51e7 100644
--- a/roles/write-devstack-local-conf/README.rst
+++ b/roles/write-devstack-local-conf/README.rst
@@ -88,3 +88,12 @@
    If a plugin declares a dependency on another plugin (via
    ``plugin_requires`` in the plugin's settings file), this role will
    automatically emit ``enable_plugin`` lines in the correct order.
+
+.. zuul:rolevar:: tempest_plugins
+   :type: list
+
+   A list of tempest plugins which are installed alongside tempest.
+
+   The list of values will be combined with the base devstack directory
+   and used to populate the ``TEMPEST_PLUGINS`` variable. If the variable
+   already exists, its value is *not* changed.