Setup branch in setup-devstack-source-dirs

Allow the setup-devstack-source-dirs role to accept a target role
to be setup - when available - for the repos.

Change-Id: Iebcba0d4be6d9d71b783e10a82c35a406afbd6bf
diff --git a/roles/setup-devstack-source-dirs/README.rst b/roles/setup-devstack-source-dirs/README.rst
index 4ebf839..4129eae 100644
--- a/roles/setup-devstack-source-dirs/README.rst
+++ b/roles/setup-devstack-source-dirs/README.rst
@@ -9,3 +9,8 @@
    :default: /opt/stack
 
    The devstack base directory.
+
+ .. zuul:rolevar:: devstack_branch
+    :default: None
+
+    The target branch to be setup (where available).