Pass active node name into ENV for hook commands

Have been using Gusteau quite a bit and enjoying it, but the default `after` hook bugged me. I tend not to have all my VMs running unless I need them, so my rspec hook would always fill up with false negatives.

This PR allows you to do the following:

```YAML
after:
  - bundle exec rspec spec/$GUSTEAU_NODE
```

and have only the relevant tests for the node you just converged/applied run.