Allow JSON as input format; use anchors for defaults not parse_config

Pros: 
- Very much resembles the output of vcloud-walker, making it a lot closer to linking our toolchain up
- Removes A LOT of code, particularly some very complex, only partially tested code for parsing and merging
- I think the input is slightly more readable with [] to indicate arrays and {} to indicates hashes, rather than dashes and spaces - but that is a matter of taste I guess.

Cons:
- Is a step back in that it's not yet DRY - but I have a plan for that, it's just a bit late right now to embark on it
- Will mean Nicola has to write his input scripts *again*. However, since he has a script to transform JSON to YAML (https://github.gds/gds/govuk-provisioning/blob/master/tools/generate_input.rb) it probably won't be as bad as rewriting it to produce yaml anchors...

I'm not averse to making this a non-breaking changes by adding a command line option to use old config for a short period of time so it doesn't break the Platform One team's workflow, as long as we understand that support for that kind of input won't be maintained. I also haven't changed the version number but what I change it to depends on the development of this before it's merged.

--- update: I've made this a non-breaking change. Now accepts YAML and JSON.