Restart postfix instead of reload after package installation (Fixes #90)

manifests/service.pp

 class postfix::service {
+  exec { 'restart postfix after packages install':
+    command     => regsubst($::postfix::params::restart_cmd, 'reload', 'restart'),
+    refreshonly => true,
+    subscribe   => Package['postfix'],
+    require     => Class['postfix::files'],
+  }
   service { 'postfix':
     ensure    => running,
     enable    => true,