Install ara on the executors

zuul v3 supports optionally collecting data via ara if it's installed.
Install it so that we can produce ara html reports for all of our runs.

The current implementation is similar to the devstack-gate
implementation - it uses a sqlite db per job. The bubblewrap invocation
sets $HOME to the work_dir, so the sqlite db for each job goes in to the
root of the work_dir with no further config.

Change-Id: Iefeda37338c57466d671ea07e7d318fa92aafc35
Depends-On: I8facdf0b95b83d43c337058d70fe6bf71e17d570

manifests/executor.pp

@@ -31,6 +31,14 @@ class zuul::executor (
     ],
   }
 
+  include ::pip::python3
+
+  package { 'ara':
+    ensure   => present,
+    provider => 'pip3',
+    require  => Class['pip'],
+  }
+
   if ($::operatingsystem == 'Ubuntu') and ($::operatingsystemrelease >= '16.04') {
     # This is a hack to make sure that systemd is aware of the new service
     # before we attempt to start it.