Conditionally install build-essential

So that the nodepool and zuul modules can be coinstalled, each needs
a conditional wrapper around the build-essential package to avoid a
duplicate resource conflict. Corresponding change is:
I17c84c21bea9a93000bc3e34f6c004c58c04c10f

Change-Id: I3f230c3a5c245fa0c2503836b5cc17b2b1106f87

manifests/init.pp

@@ -56,7 +56,6 @@ class nodepool (
   }
 
   $packages = [
-    'build-essential',
     'libffi-dev',
     'libssl-dev',
     'libgmp-dev',         # transitive dep of paramiko
@@ -69,6 +68,12 @@ class nodepool (
     ensure  => present,
   }
 
+  if ! defined(Package['build-essential']) {
+    package { 'build-essential':
+      ensure => present,
+    }
+  }
+
   file { '/etc/mysql/conf.d/max_connections.cnf':
     ensure  => present,
     content => "[server]\nmax_connections = 8192\n",