[vsphere] allow setting ram and num of cpu when cloning

builds a `virtual_machine_config_spec` with memoryMB + numCPUs when cloning if they are passed in as options.

eg:

``` ruby
#!/usr/bin/env ruby

require 'fog'
require 'pp'

connection = Fog::Compute[:vsphere]

options = { 
  'datacenter'          => 'syd07',
  'dest_folder'         => 'foo',
  'template_path'       => 'templates/ubuntu-precise-amd64',
  'name'                => 'bar-web-11',
  'resource_pool'       => ['cluster1','resource pool 2'],
  'memoryMB'            => '8192',
  'numCPUs'             => 4,
  'power_on'            => true,
  'wait'                => true
}

new_vm=connection.vm_clone(req_options)
pp new_vm

```