Add beaker acceptance tests

This adds beaker acceptance tests.  There are two possible machine
images. You can run them with the commands:

    BEAKER_set=centos-64-x64 bundle exec rake beaker
    BEAKER_set=ubuntu-1204-x64 bundle exec rake beaker

The machine names correspond to files in
spec/acceptance/nodesets/*.yml. If you don't specify a `BEAKER_set`, it
will look for one called `default`.

I added a basic test to check the default functionality --
carbon-aggregator not running, carbon-cache running and listening on
2003, graphite-web running and listening on 127.0.0.1:8000.

The tests don't pass on centos (as expected at the moment), and I
haven't added them to travis in this commit.
This might be easier to read if indented as (note hyphen before heredoc):
```ruby
  it 'should run successfully' do
    pp = <<-END
      class { 'python':
        pip        => true,
        dev        => true,
        virtualenv => true,
      }

      class { 'graphite':
        require => Class['python'],
      }
    END

    # Apply twice to ensure no errors the second time.
```