Make localuser router optional

Under some circumstances it can be useful to not include the default
"localuser" Exim router. Make it optional via a
default_localuser_router class parameter which defaults to true,
preserving backward compatibility.

Change-Id: Iae38041c95bc0bbcd1dfd3d0c5c626b63acd7922

manifests/init.pp

@@ -27,6 +27,7 @@ class exim(
   $queue_run_max            = '5',
   $queue_smtp_domains       = undef,
   $routers                  = [],
+  $default_localuser_router = true,
   $smarthost                = false,
   $sysadmins                = [],
   $transports               = [],

templates/exim4.conf.erb

@@ -652,6 +652,7 @@ system_aliases:
 
 # .forward files are not supported --jeblair
 
+<% if @default_localuser_router == true -%>
 # This router matches local user mailboxes. If the router fails, the error
 # message is "Unknown user".
 
@@ -667,6 +668,7 @@ localuser:
 # local_part_suffix_optional
   transport = local_delivery
   cannot_route_message = Unknown user
+<% end -%>
 
 <% if @mailman_domains.length > 0 -%>
 # Pick up on messages from our local mailman and route them via our