enable setting python version for ensure-twine

Add twine_python so we can control the version of python used to
install twine.

Change-Id: I36ddbdf78f549ddc022cce86f3ada9631fb19c37
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

roles/ensure-twine/README.rst

@@ -1 +1,9 @@
 Ensure twine is installed.
+
+**Role Variables**
+
+.. zuul:rolevar:: twine_python
+   :default: python
+
+   The python interpreter to use to install twine if it is not already
+   installed. Set it to "python3" to use python 3, for example.

roles/ensure-twine/defaults/main.yaml

@@ -0,0 +1,2 @@
+---
+twine_python: python

roles/ensure-twine/tasks/main.yaml

@@ -11,7 +11,7 @@
 - name: Ensure twine is installed
   block:
     - name: Ensure twine is installed
-      command: pip install twine!=1.12.0 readme_renderer[md]!=23.0 --user
+      command: "{{ twine_python }} -m pip install twine!=1.12.0 readme_renderer[md]!=23.0 --user"
 
     - name: Set pypi_twine_executable
       set_fact: