machine.current_driver should returns the current driver from the chef_metal run context if it exists otherwise it uses the machine driver url to new one up

As discussed in Issue #62, I have been having issues with duplicate convergances after bootstrapping with chef metal. The second convergance fails with: 

```
ArgumentError: provisioner_cluster[CHEF] (provisioner::chef_server line 6) had an error: ArgumentError: machine[QA1T3NDEVOPSLABCHEF01] (/home/mwrock/.chef/local-mode-cache/cache/cookbooks/provisioner/providers/cluster.rb line 30) had an error: ArgumentError: Missing required arguments: vsphere_username, vsphere_password
```

The first convergance is invoked from the machine_batch which uses the config vlaues stached in the run context via:
```
  with_vsphere_driver :vsphere_server => hypervisor['vsphere_server'],
    :vsphere_username => hypervisor['vsphere_username'],
    :vsphere_password => hypervisor['vsphere_password'],
    :vsphere_expected_pubkey_hash => hypervisor['vsphere_expected_pubkey_hash'],
    :ssh_password => node['provisioner']['ssh_password']
```
but the second run invoked by the Machine resource calls:

```
run_context.chef_metal.driver_for_url(machine_spec.driver_url)
```
to get the driver and thus my driver options are lost since they are not in the client config.

While the deeper issue is why I see duplicate convergances, it seems like calls to machine.current_driver should reuse the driver in the runcontext.