Fix duplicated package for build-essential

When installing nodepool on single_node_ci context, it
gives an error for build-essential duplicated declaration
(nodepool and zuul). Zuul is protecting that with an if !defined
declaration, but nodepool is not doing that.
Enclose all the package list in ensure_packages, to protect
from duplication errors.

Change-Id: Ibfc7af81ff8429d592d7d9fe70b06e1f5be4e77f

manifests/init.pp

@@ -65,9 +65,7 @@ class nodepool (
     'libxslt-dev',
   ]
 
-  package { $packages:
-    ensure  => present,
-  }
+  ensure_packages($packages, {'ensure' => 'present'})
 
   file { '/etc/mysql/conf.d/max_connections.cnf':
     ensure  => present,