Fix pip ordering and build requirements

When bootstrapping a new server it's noticed that pip wasn't installed
before trying to use it.  Add it as a dependency of the pip installs.

It was installed by the time the whisper install ran, but there no gcc
required for some of the dependencies.  Add build-essential.

Change-Id: I29af4665a1ef7c6f78d9fae29db066ea64743d70

manifests/init.pp

@@ -69,7 +69,8 @@ class graphite(
     path        => '/usr/local/bin:/usr/bin:/bin',
     refreshonly => true,
     subscribe   => Vcsrepo['/opt/graphite-web'],
-    require     => [Exec['install_carbon'],
+    require     => [Class['pip'],
+                    Exec['install_carbon'],
                     File['/var/lib/graphite/storage']]
   }
 
@@ -87,7 +88,8 @@ class graphite(
     path        => '/usr/local/bin:/usr/bin:/bin',
     refreshonly => true,
     subscribe   => Vcsrepo['/opt/carbon'],
-    require     => [Exec['install_whisper'],
+    require     => [Class['pip'],
+                    Exec['install_whisper'],
                     File['/var/lib/graphite/storage']]
   }
 
@@ -98,11 +100,21 @@ class graphite(
     source   => 'https://github.com/graphite-project/whisper.git',
   }
 
+  if ! defined(Package['build-essential']) {
+    package { 'build-essential':
+      ensure => present,
+    }
+  }
+
   exec { 'install_whisper' :
     command     => 'pip install /opt/whisper',
     path        => '/usr/local/bin:/usr/bin:/bin/',
     refreshonly => true,
     subscribe   => Vcsrepo['/opt/whisper'],
+    require     => [
+      Class['pip'],
+      Package['build-essential'],
+    ],
   }
 
   user { 'statsd':