Define collectd plugins separately for Bluebox.

As CPUs show up for the vserver host, it's safe to remove them.
They won't give us anything meaningful in this case.

worker_host/travis_worker_collectd/recipes/bluebox.rb

@@ -1,4 +1,12 @@
-include_recipe "travis_worker_collectd"
+require_recipe 'collectd'
+
+collectd_plugin "load"
+
+collectd_plugin "memory"
+
+collectd_plugin "interface" do
+  options :interface => "lo", :ignore_selected => true
+end
 
 collectd_plugin "df" do
   options :mount_point => "/",