Add option to set delete-old imap messages

manifests/server.pp

@@ -21,6 +21,7 @@ class lpmqtt::server (
   $imap_password,
   $imap_use_ssl = true,
   $imap_folder = 'INBOX',
+  $imap_delete_old = false,
   $mqtt_hostname = 'firehose01.openstack.org',
   $topic = 'launchpad',
   $mqtt_username = 'infra',

templates/lpmqtt.conf.erb

@@ -4,6 +4,7 @@ username = <%= @imap_username %>
 password = <%= @imap_password %>
 use_ssl = <%= @imap_use_ssl %>
 folder = <%= @imap_folder %>
+delete-old = <%= @imap_delete_old %>
 
 [mqtt]
 hostname = <%= @mqtt_hostname %>