Idea for plugin config

From within the roles.yml I'd like to be able to configure specific plugins. E.g. in Vagrantfile:
```
if Vagrant.has_plugin?("vagrant-vbguest") then
  config.vbguest.auto_update = false
end
```
In the roles.yml this could translate to something like e.g.:
```
---
  some-box:
    private_networks:
      - {ip: '0.0.0.0', auto_network: true}
    forwarded_ports:
      - {guest: 2375, host: 2375, auto_correct: true}
    provider:
      type: virtualbox
      check_guest_additions: false
      functional_vboxsf: false
      customize:
        - [modifyvm, !ruby/sym id, '--memory', 256]
        - [modifyvm, !ruby/sym id, '--natdnshostresolver1', 'on']
        - [modifyvm, !ruby/sym id, '--natdnsproxy1', 'on']
        - [modifyvm, !ruby/sym id, '--uart1', '0x3F8', '4']
    provisioners:
      - {type: hosts}
    plugins:
      - {plugin: vagrant-vbguest, config_attribute: vbguest, { auto_update: false } }
```
This pull request contains an untested (!) idea of what I'm trying to achieve, however my Ruby coding stinks... 

Would this be achievable?