fetch-zuul-cloner: use ensure-pip

Firstly, the "destination" variable was replaced with a virtualenv in
6bb1f6046b692ee2038eafd0965bd3095d6fa787; update the documentation and
prefix the name as is now standard for variables in this repo.  This
is not set anywhere so I think the prefixing is OK.

Call ensure-pip before installing, and use the virtualenv_command it
exports to create the environment for the zuul-cloner binary.

Story: #2007386
Task: #39311
Change-Id: I057191bf20ab1650a8926971dd386ce9cb1f03f5

roles/fetch-zuul-cloner/README.rst

@@ -1,10 +1,13 @@
 Fetch the zuul-cloner shim and install to the destination.
 
+**Role Variables**
+
 .. zuul:rolevar:: repo_src_dir
 
    Location of the Zuul source repositories.
 
-.. zuul:rolevar:: destination
+.. zuul:rolevar:: fetch_zuul_cloner_virtualenv
+   :default: ``/usr/zuul-env``
 
-   Where to install the zuul-cloner shim. This should be the full path
-   and filename.
+   The path to the virtualenv to install the shim.  See the ensure-pip
+   role for details of virtualenv creation.

roles/fetch-zuul-cloner/defaults/main.yaml

@@ -1 +1 @@
-virtualenv: /usr/zuul-env
+fetch_zuul_cloner_virtualenv: /usr/zuul-env

roles/fetch-zuul-cloner/tasks/main.yaml

@@ -1,18 +1,23 @@
+- name: Run ensure-pip
+  include_role:
+    name: ensure-pip
+
 - name: Install zuul-cloner shim dependencies
   pip:
     name: PyYAML
-    virtualenv: "{{ virtualenv }}"
+    virtualenv_command: '{{ ensure_pip_virtualenv_command }}'
+    virtualenv: '{{ fetch_zuul_cloner_virtualenv }}'
   become: yes
 
 - name: Install zuul-cloner shim
   template:
     src: templates/zuul-cloner-shim.py.j2
-    dest: "{{ virtualenv }}/bin/zuul-cloner"
+    dest: "{{ fetch_zuul_cloner_virtualenv }}/bin/zuul-cloner"
   become: yes
 
 - name: Change zuul-cloner permissions
   file:
-    path: "{{ virtualenv }}/bin/zuul-cloner"
+    path: "{{ fetch_zuul_cloner_virtualenv }}/bin/zuul-cloner"
     mode: 0755
   become: yes