Add custom options to node definition

### Summary
Extends nodeset.yml to support new options to customize the VM. Supports enhancement (#44)

Adds options:
- **mac**: Custom mac-address (Randomized if left blank)
- **cpus**: Sets the number of virtual cpus
- **memory**: Sets the node memory (in mb)

**Note**: Currently only support Vagrant + VirtualBox by adding customizations to the Vagrantfile. 
I'd welcome any patches that can add these customizations to VSphere too.

### Example nodeset.yml
```yaml
---
default_set: 'centos-58-x64'
sets:
  'centos-58-x64':
    nodes:
      "master":
        prefab: 'centos-58-x64'
        options:
          mac: 9DC6D29FA536
          cpus: 2
          memory: 4096 #mb
      "agent":
        prefab: 'centos-58-x64'
        options:
          mac: 9DC6D29FA537
          cpus: 1
          memory: 768 #mb
```