Install jemalloc on nodepool servers

We already use this library on the Zuul executors to help with
memory issues. The nodepool builders and launchers are seeing
memory issues as well, and it is believed/hoped that using jemalloc
will help reduce some of the memory pressure.

Change-Id: I02129bb33baf00fedb5d1a6f6b82944a05c84963

manifests/init.pp

@@ -96,6 +96,13 @@ class nodepool (
     }
   }
 
+  # Alternative malloc implementation with better packing performance
+  if ! defined(Package['libjemalloc1']) {
+    package { 'libjemalloc1':
+      ensure => present,
+    }
+  }
+
   user { 'nodepool':
     ensure     => present,
     home       => '/home/nodepool',