Allow own Xtrabackup script

We have our own backup script with some additional features, of which
quite a few are specific for our platform. I could add some of them
here as well, but this would end up in a completely new script.
That's why I try it with this minimally invasive variant :)

Author: SaschaDoering

manifests/backup/xtrabackup.pp

@@ -14,6 +14,7 @@
   $backupdirgroup          = $mysql::params::root_group,
   $backupcompress          = true,
   $backuprotate            = 30,
+  $backupscript_template   = 'mysql/xtrabackup.sh.erb',
   $ignore_events           = true,
   $delete_before_dump      = false,
   $backupdatabases         = [],
@@ -94,6 +95,6 @@
     mode    => '0700',
     owner   => 'root',
     group   => $mysql::params::root_group,
-    content => template('mysql/xtrabackup.sh.erb'),
+    content => template($backupscript_template),
   }
 }