Fix the reference_v30_value / reference_vs30_value name consistency issue

[larsbutler]

There is a mismatch in this value between the client and most of our backend code. We're having to deal with 'translating' this value (for example, in geonode.mtapi.views.prepare_job()).

We need to implement a permanent solution (fix the inconsistent naming) and remove these translation workarounds.