puppet: Add a daily cron-job to send zulip update announcements.

A daily cron job is configured to run the
'send_zulip_update_announcements' management command.

puppet/zulip/files/cron.d/send_zulip_update_announcements

@@ -0,0 +1,6 @@
+SHELL=/bin/bash
+PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
+USER=zulip
+
+# Cron job to send zulip update announcements hourly, if there are any.
+47 * * * * zulip /home/zulip/deployments/current/manage.py send_zulip_update_announcements

puppet/zulip/manifests/app_frontend_once.pp

@@ -74,4 +74,12 @@ class zulip::app_frontend_once {
     mode   => '0644',
     source => 'puppet:///modules/zulip/cron.d/clearsessions',
   }
+
+  file { '/etc/cron.d/send_zulip_update_announcements':
+    ensure => file,
+    owner  => 'root',
+    group  => 'root',
+    mode   => '0644',
+    source => 'puppet:///modules/zulip/cron.d/send_zulip_update_announcements',
+  }
 }