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

[mkuzmin]

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 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.

[marnovdm]

Could this be made optional? We already deploy our VMs in a folder named after the Vagrant project using the vsphere.vm_base_path option, so having the project name also prepended to the VMs we deploy seems a bit redundant and looks 'less clean' in my opinion.

[mkuzmin]

This change was a kind of bugfix. The plugin had a non-standard behaviour, and I made it work the same way the core Vagrant does.
vm_base_path is an optional parameter. Yes, VMs can be sorted into different folders, but this not a default case.

I understand your point, and think a new option like vsphere.skip_name_prefix would be valuable, but please do not revert this change, and leave a default behavior as it is today.