Install libgmp-dev on nodepool

Required in order for pycrypto to be anything other than
extremely slow.

Change-Id: I8c9b2a6f9f52db80c8b9fbf033e1843ff3ee342e

modules/nodepool/manifests/init.pp

@@ -78,6 +78,7 @@ class nodepool (
     'libssl-dev',
     'kpartx',
     'qemu-utils',
+    'libgmp-dev',         # transitive dep of paramiko
   ]
 
   package { $packages:
@@ -133,6 +134,7 @@ class nodepool (
       Package['libffi-dev'],
       Package['libssl-dev'],
       Package['python-lxml'],
+      Package['libgmp-dev'],
     ],
   }