boot2docker distribution was removed from the prlctl v19+

[AlexanderSkogorev]

This line tries to create a VM using boot2docker distribution. This type of distribution is not available anymore at the plctl v19+:

docker-machine-parallels/parallels_darwin.go

Line 114 in 5a11c2b

"--distribution", "boot2docker",

I came here from the minikube issue: kubernetes/minikube#18196
Minikube uses this driver to work with parallels.