Ensure virtualenv is on the destination host

The virtualenv package is required in order to
both prepare a virtualenv in the build stage,
and to ensure that the right python binary is
in the virtualenv in the install stage.

This patch ensures that the virtualenv is
installed on the destination host.

tasks/python_venv_preflight.yml

@@ -43,3 +43,13 @@
   run_once: yes
   when:
     - venv_reuse_enable | bool
+
+- name: Ensure that virtualenv is installed on the destination host
+  pip:
+    name: virtualenv
+    state: latest
+    extra_args: "{{ host_pip_install_args }}"
+  register: _install_host_pip_virtualenv
+  until: _install_host_pip_virtualenv | success
+  retries: 5
+  delay: 2