ensure-twine: make python3 default, ensure pip installed

Make the default twine python installation use "python3".  Use the
ensure-pip role before calling pip.

This role currently isn't tested; it is added with upload-pypi testing
in the follow-on Ie40ec32b3b2ffc4646301c6e145c527fe6a3bae2

Change-Id: I55ef79bf9c5af9e4d1da24ed654821277edb663b

roles/ensure-twine/defaults/main.yaml

@@ -1,3 +1,3 @@
 ---
-twine_python: python
+twine_python: python3
 twine_executable: twine

roles/ensure-twine/tasks/main.yaml

@@ -13,6 +13,10 @@
 
 - name: Ensure twine is installed
   block:
+    - name: Ensure pip
+      include_role:
+        name: ensure-pip
+
     - name: Ensure twine is installed
       command: "{{ twine_python }} -m pip install twine!=1.12.0 requests-toolbelt!=0.9.0 --user"