[Reviewer: Felix] Fix use_subdomain again - the use_subdomain key is a string, not a symbol

Felix, please can you review?  I was seeing a problem where the top-level DNS domains were being updated rather than my environment-specific subdomains.  The issue was that the use_subdomain flag was not being picked up.  I'm not completely confident of this fix (my chef knowledge is a bit limited) but it does seem to work.