CI: give enough time to the promote job

It takes time to download and upload that 2GB file from the object
storage. Previously, it took 2mins, which is too close to that default
timeout.

Change-Id: I1c2c61e40ec20664597d9d7525ad78148517e803
diff --git a/.zuul.yaml b/.zuul.yaml
index 208ce46..bec23c9 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -49,4 +49,5 @@
 
     promote:
       jobs:
-        - promote-artifacts
+        - promote-artifacts:
+            timeout: 600