Make /etc/aliases extensible

In an effort to support customizing E-mail aliases per server, allow
a hash of aliases to be passed into the /etc/aliases template. While
we're here, remove the custom gerrit2 and jenkins aliases since
those can now be set individually on servers where they actually
matter rather than unnecessarily setting them everywhere.

Change-Id: I2911f157812c127a514196ae58b7609378d7d4e4

manifests/init.pp

@@ -32,6 +32,7 @@ class exim(
   $transports               = [],
   $smtp_accept_max          = undef,
   $smtp_accept_max_per_host = undef,
+  $extra_aliases            = {},
 ) {
 
   include ::exim::params

templates/aliases.erb

@@ -1,4 +1,8 @@
 # /etc/aliases
+<% @extra_aliases.each do |key,value| -%>
+<%= key %>: <%= value %>
+<% end -%>
+
 mailer-daemon: postmaster
 postmaster: root
 nobody: root
@@ -12,8 +16,6 @@ abuse: root
 noc: root
 security: root
 
-gerrit2: root
-jenkins: root
 <% if @sysadmins.length > 0 -%>
 root: <%= @sysadmins.join(",") %>
 <% end -%>