contrib/vagrant: Allow to start a single VM
contrib/vagrant: Allow to start a single VM

This change allows to run start.sh with a positional argument with the
name of the machine to start. The use case is to be able to join more
nodes to the cluster.

Example:

  export NWORKERS=0
  ./contrib/vagrant/start.sh

And then to add the worker separately:

  export NWORKERS=1
  ./contrib/vagrant/start.sh k8s2

Signed-off-by: Michal Rostecki <82ebf970c69340aebb770c3853be5fd96a3eb584@opensuse.org>

tgraf/cilium,tgraf/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium