Set project merge-mode to rebase

This change sets the merge mode used by Git in this project.
The mode should match the remote system, that in this project is
GitHub.
"rebase" mode, rebase the changes onto the branch when merging.

Author: fserucas

.zuul.yaml

@@ -1,5 +1,6 @@
 ---
 - project:
+    merge-mode: rebase
     check:
       jobs:
         - pre-commit