Install pbr in pip3 role to fix issue xenial

When installing bots on eavesdrop with py27/py35, there is a
regression with the latest pbr release. The workaround is to
have pbr preinstalled in the pip3 role.

Change-Id: I5ea790a50e180df36b480dcbb13530a80f398b5e

playbooks/roles/pip3/tasks/main.yaml

@@ -39,3 +39,13 @@
   loop:
     - pip
     - virtualenv
+
+# TODO(frickler): pbr is being installed as a workaround for issues
+# with eavesdrop running on xenial with py27/35. It can be dropped
+# once the host has been upgraded
+- name: Install latest pbr on xenial
+  pip:
+    name: pbr
+    state: latest
+    executable: pip3
+  when: ansible_distribution_release == 'xenial'