Add pypi-pre-commands to allow build steps before pypi publish

Change-Id: I1d564a5ef93b893f2767ee1dae9ccc60ab826bf2
diff --git a/jjb/verify/voltha-protos.yaml b/jjb/verify/voltha-protos.yaml
index ada8bd0..0f193ab 100644
--- a/jjb/verify/voltha-protos.yaml
+++ b/jjb/verify/voltha-protos.yaml
@@ -11,6 +11,7 @@
       - 'post-merge-voltha-protos-jobs':
           branch-regexp: '{supported-branches-regexp}'
           pypi-index: 'pypi'
+          pypi-prep-commands: 'make python-protos'
 
 - job-group:
     name: 'verify-voltha-protos-jobs'