Align exim module parameter name with the tree

The rest of the tree, literally everwhere, calls this value
sysadmins - but the exim module calls it sysadmin. This doesn't
really matter of course, but I noticed it while looking in to a
review on another patch and couldn't not fix it. Also, there are
two comments that are clearly remnants from having used exim.pp
as a starting point.

Change-Id: I184701c8c4d4b1e9d93d746b2ad0c5fc53443866

manifests/init.pp

@@ -4,7 +4,7 @@ class exim(
   $queue_run_max = '5',
   $queue_smtp_domains = '',
   $smarthost = false,
-  $sysadmin = []
+  $sysadmins = []
 ) {
 
   include exim::params

templates/aliases.erb

@@ -14,6 +14,6 @@ security: root
 
 gerrit2: root
 jenkins: root
-<% if sysadmin.length > 0 -%>
-root: <%= sysadmin.join(",") %>
+<% if sysadmins.length > 0 -%>
+root: <%= sysadmins.join(",") %>
 <% end -%>