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

Change-Id: I1d564a5ef93b893f2767ee1dae9ccc60ab826bf2
diff --git a/jjb/pypi-publish.yaml b/jjb/pypi-publish.yaml
index b2076e9..0f6aca9 100644
--- a/jjb/pypi-publish.yaml
+++ b/jjb/pypi-publish.yaml
@@ -44,6 +44,7 @@
           properties-content: |
             PYPI_INDEX={pypi-index}
             PYPI_MODULE_DIRS={pypi-module-dirs}
+            PYPI_PREP_COMMANDS={pypi-prep-commands}
 
       - shell: !include-raw-escape: shell/pypi-publish.sh