Install latest diskimage-builder on nodepool

Keep nodepool's version of diskimage-builder up-to-date by installing
the latest version for pip. DIB releases at least weekly so we can get
update quickly via this install method.

Change-Id: I7e97388d890d37f1f86b860c2309ab1d63f76e41

manifests/init.pp

@@ -42,6 +42,13 @@ class nodepool (
     }
   }
 
+  # required by the nodepool diskimage-builder element scripts
+  if ! defined(Package['python-yaml']) {
+    package { 'python-yaml':
+      ensure => present,
+    }
+  }
+
   class { 'mysql::server':
     config_hash => {
       'root_password'  => $mysql_root_password,
@@ -102,6 +109,15 @@ class nodepool (
     source   => $git_source_repo,
   }
 
+  package { 'diskimage-builder':
+    ensure   => latest,
+    provider => pip,
+    require  => [
+      Class['pip'],
+      Package['python-yaml'],
+    ],
+  }
+
   include pip
   exec { 'install_nodepool' :
     command     => 'pip install /opt/nodepool',