Add optional VM name parameter to make it possible to override 'default'.

Sometimes is useful to change the VM name created by vagrant within the hypervisor. You may need to specify different VM names based on the provider.

So... just like, for example, on virtualbox provider you can do this:
```ruby
config.vm.provider 'virtualbox' do |vbox|
    vbox.name = 'rickroll'
end
```

With this PR, you should be able to do this as well:
```ruby
config.vm.provider 'vcloud' do |vcloud|
    vcloud.name = 'rickroll'
end
```