Reviewer MIRW: Add support for mangelwurzel node

Hi Matt,

Please can you review my chef changes to allow us to spin up mangelwurzel nodes?

I've managed to successfully run the following test plan:
* Run knife box create -E <env> mangelwurzel
* Check a new node is created at mangelwurzel.<env>.cw-ngv.com with the mangelwurzel-as plugin installed on top of sprout-base
* Check it can receive SIP traffic on port 5058 from sprout
* Run knife box delete -E <env>-mangelwurzel and check the node has been deleted

Thanks,
Graeme