Add ensure-python role

ensure-python allows for installation of a specific version of
the python interpreter and development files. Add this role to
tox pre.yaml.

Change-Id: Ia56cba58c87aa48de3e8e3f3d28f6252f7dcd7c3

playbooks/tox/pre.yaml

@@ -1,3 +1,4 @@
 - hosts: all
   roles:
     - ensure-tox
+    - ensure-python

roles/ensure-python/README.rst

@@ -0,0 +1,10 @@
+Ensure specified python interpreter and development files are installed
+
+.. note:: This role is only available for Debian based platforms
+          currently.
+
+**Role Variables**
+
+.. zuul:rolevar:: python_version
+
+  Optional version of python interpreter to install, such as ``3.7``.

roles/ensure-python/tasks/main.yaml

@@ -0,0 +1,6 @@
+- name: Install specified version of python interpreter and development files
+  when: python_version is defined
+  package:
+    name: python{{ python_version }}-dev
+    state: present
+  become: yes