Zuul: add run attribute

A proposed change[1] to Zuul removes the implied run attribute.
Add an explicit run attribute here to prepare for that.

[1] Ia8f23bce9898cd4f387554e6787b091b63e75519

Change-Id: I1fbc36c3d1b8c4ed70fceef1c587255dad50da04
diff --git a/.zuul.yaml b/.zuul.yaml
index 9e00231..826410c 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -71,6 +71,7 @@
         horizon: False
         tempest: False
     pre-run: playbooks/pre
+    run: playbooks/devstack
     post-run: playbooks/post