Set/Change cpus and memory at power on

Here is a 'little' PR to set / change the number of CPUs and memory size of a VM at power on. I have added the config keywords `cpus` and `memory` in the same way as Vagrant has them for the default VirtualBox provider. See [the docs](http://docs.vagrantup.com/v2/virtualbox/configuration.html) to compare the configuration.

With that change it is possible to customize CPUs and memory in Vagrantfile like this:

```ruby
    config.vm.provider "vcloud" do |v|
      v.memory = 4096
      v.cpus = 4
    end
```

On every power on the new function compares the current values of the VM with the values given in Vagrantfile. If there are values in Vagrantfile and if one of these values differ, then the hardware settings are changed before power on. Otherwise no task will be created to keep power on fast as before.

Hope you might find it useful ;-)