Use a directory name where Vagrantfile is stored as a prefix for VM name

At the moment all virtual machines created on vSphere get names with `default_` prefix. That gets hard to navigate and manage them, when many machines are running in parallel.
A [workaround](http://stackoverflow.com/a/20431791/576361) exists to rename machines from `default` to other values, but it requires additional modifications in each Vagrantfile individually, and doesn't allow to set a default prefix for all instances of the box.

When Vagrant creates VMs under VirtualBox, it uses the current directory name where `Vagranfile` is stored as a prefix.
This PR adds the same logic here.