Allow accessing of attributes within the namespace

@sethvargo 

When I started replacing my taps with this, I lost the feature of being able to access attributes within the tap namespace.

For example
```ruby
node.test.tap do |test|
  test['new_attribute'] = File.join(test['existing_attribute'], 'bin')
end
```
would work, but
```ruby
node.namesepace 'test' do
  new_attribute File.join(existing_attribute, 'bin')
end
```
Wouldn't. This PR would allow it to work.
Instead of doing this, just generate a compiled attribute and check its value:

```ruby
namespace :foo do
  namespace :bar do
    zip 'zap'
    blitz "#{zip}zoo"
  end
end
```

```ruby
expect(node['foo']['bar']).to eq('zapzoo')
```