Use ensure-pip for stackviz pip install

Infra is trying to move to nodes that do not come with pip/virtualenv
pre-installed on the image; for details see the spec at [1].

zuul-jobs include ensure-pip [2] to provide pip dependencies; use that
before installing stackviz.  Note that if pip is installed this is a
noop.

[1] https://docs.opendev.org/opendev/infra-specs/latest/specs/cleanup-test-node-python.html
[2] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#role-ensure-pip

Needed-By: https://review.opendev.org/723762
Change-Id: I24925ab8e885404da64a3ecfca3848041b6dfc05

roles/process-stackviz/tasks/main.yaml

@@ -17,13 +17,18 @@
   when: not subunit_input.stat.exists
 
 - name: Install stackviz
-  pip:
-    name: "file://{{ stackviz_archive.stat.path }}"
-    virtualenv: /tmp/stackviz
-    extra_args: -U
   when:
     - stackviz_archive.stat.exists
     - subunit_input.stat.exists
+  block:
+    - include_role:
+        name: ensure-pip
+
+    - pip:
+        name: "file://{{ stackviz_archive.stat.path }}"
+        virtualenv: /tmp/stackviz
+        virtualenv_command: '{{ ensure_pip_virtualenv_command }}'
+        extra_args: -U
 
 - name: Deploy stackviz static html+js
   command: cp -pR /tmp/stackviz/share/stackviz-html {{ stage_dir }}/stackviz