Refactor: migrate testapp to nlohmann::json [1/n]
Refactor: migrate testapp to nlohmann::json [1/n]

Generate the configuration object by using nlohmann::json. To
minimize the size of the patch keep the external interface
of the generate method.

Change-Id: Ica6eb3ea994c08b801805aae9f0a52cfaf29d682
Reviewed-on: http://review.couchbase.org/100995
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine