Add devstack base job for zuul v3

This should be managed in the devstack repo, since it's a base job to
run devstack.

Change-Id: Iffe54fbccbccd68db08f79a1b51dd7f76dbff408
Depends-On: Ie2119f24360d56690ffd772b95a9ea6b98dd4a39
diff --git a/roles/setup-devstack-source-dirs/defaults/main.yaml b/roles/setup-devstack-source-dirs/defaults/main.yaml
new file mode 100644
index 0000000..fea05c8
--- /dev/null
+++ b/roles/setup-devstack-source-dirs/defaults/main.yaml
@@ -0,0 +1 @@
+devstack_base_dir: /opt/stack