Install python3-venv for venv creation on mirror-update

We moved afsmon installation to a virtualenv using python3 -m venv but
apparently don't have the python3-venv package installed. Fix that so
that we can create the virtualenv.

Change-Id: I4f33b3e2b626298d1cd6a7229af03a2c89c0ed9a

playbooks/roles/afsmon/tasks/main.yaml

@@ -6,6 +6,12 @@
     group: root
     mode: 0755
 
+- name: Install venv package
+  package:
+    name:
+      - python3-venv
+    state: present
+
 - name: Install afsmon to dedicated venv
   pip:
     name: afsmon