Unnecessary json.dumps in data and dns plugins

[JeremyRand]

Several of the methods in the data and dns plugins are returning a string (generated using json.dumps) instead of simply returning the JSON data itself. This seems completely unnecessary, and makes it more complicated for applications (or people!) to query NMControl. While fixing this will break existing client applications until they're patched (the patch is trivial), I think we should fix this sooner rather than later. (And I speak as the maintainer of at least two applications that will need patching.) Notably, data.getValueProcessed is doing this the correct way already.

If we can get an ACK on the desirability of fixing this, I'm happy to implement the fix.

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