Supports setting rabbitmq config file to a different path

issue #157
Fixes issue #157
How about below?
```
     let(:runner) { ChefSpec::ServerRunner.new(REDHAT_OPTS) }
     let(:node) { runner.node }
     let(:chef_run) do
       node.set['rabbitmq']['config'] = '/path/to/rabbitmq'
       runner.converge(described_recipe)
     end
```