allow build-python-release consumers to override python version

Define a release_python variable that users of the
build-python-release role can override to set which version of python
they are using for building sdists and wheels.

Change-Id: I1502c572304000fab45a43e1a6d674f5aa7e9a9e
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

roles/build-python-release/README.rst

@@ -0,0 +1,9 @@
+Build sdist and wheel for Python projects.
+
+** Role Variables **
+
+.. zuul:rolevar:: release_python
+   :default: python
+
+   The python interpreter to use. Set it to "python3" to use python 3,
+   for example.

roles/build-python-release/defaults/main.yaml

@@ -1 +1,2 @@
 zuul_work_dir: "{{ zuul.project.src_dir }}"
+release_python: "python"

roles/build-python-release/tasks/main.yaml

@@ -1,4 +1,4 @@
 - name: Build a tarball and wheel
-  command: python setup.py sdist bdist_wheel
+  command: "{{release_python}} setup.py sdist bdist_wheel"
   args:
     chdir: "{{ zuul_work_dir }}"