Let Puppet manage make.conf

There is no need to pass build options from the CI host because the
algorithm Puppet uses to make /etc/portage/make.conf works just as well
inside the container.
5 jobs for master in 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
canceled #328
nest
Build haswell-server

00:00:58

failed #322
nest
Build haswell-server

00:02:41

failed #323
nest
Build haswell-server

00:11:02

failed #324
nest
Build haswell-server

00:02:47

passed #325
nest
Build haswell-server

00:16:10