Fix puppet-lint issues

There were some issues with the init.pp module with puppet-lint which
weren't fixed before the repo was imported into gerrit. Now that we have
ci these issues were blocking changes. This commit fixes those.

Change-Id: I2c619c8c3fa55cabe6325ee48f7a531f097b3cc0

manifests/init.pp

@@ -14,7 +14,7 @@ class mqtt_statsd(
 ){
   include ::pip
 
-  vcsrepo { "$source_dir" :
+  vcsrepo { $source_dir :
     ensure   => latest,
     provider => git,
     revision => $git_revision,
@@ -22,10 +22,10 @@ class mqtt_statsd(
   }
 
   exec { 'install_mqtt_statsd' :
-    command     => "pip install -U $source_dir",
+    command     => "pip install -U ${source_dir}",
     path        => '/usr/local/bin:/usr/bin:/bin/',
     refreshonly => true,
-    subscribe   => Vcsrepo["$source_dir"],
+    subscribe   => Vcsrepo[$source_dir],
     require     => Class['pip'],
   }