Use python3 for building releases. Everywhere.

We set this for the check and gate pipelines but not release which
made us very surprised and sad.

We made project vars to avoid this problem.  This patch uses them.

Change-Id: Ia64323e2222b5a2f442eceb310172826556b0c1a

.zuul.yaml

@@ -336,6 +336,8 @@
           DIB_SIMPLE_INIT_NETWORKMANAGER: '1'
 
 - project:
+    vars:
+      release_python: python3
     check:
       jobs:
         - opendev-buildset-registry
@@ -361,9 +363,7 @@
         - zuul-quick-start:
             requires: zuul-container-image
             dependencies: nodepool-build-image
-        - build-python-release:
-            vars:
-              release_python: python3
+        - build-python-release
     gate:
       jobs:
         - nodepool-upload-image
@@ -379,9 +379,7 @@
         - zuul-quick-start:
             requires: zuul-container-image
             dependencies: nodepool-upload-image
-        - build-python-release:
-            vars:
-              release_python: python3
+        - build-python-release
     promote:
       jobs:
         - nodepool-promote-image