Refactor Machinery RPM injection into a vagrant provisioner plugin

The old method of just calling a helper method from within the VM
definition blocks had the big downside that the RPM was build every time
the Vagrantfile was evaluated, also when running "vagrant destroy", for
example.

We now have a vagrant provisioner plugin instead which can be activated
for the machinery VMs by adding the "machinery_rpm" provisioner like
this:

  machinery_131.vm.provision "machinery_rpm"

or this, when overriding the obs configuration:

  machinery_sles.vm.provision "machinery_rpm",
    :api     => "https://your.buildservice.com",
    :project => "machinery_project",
    :package => "machinery"