rgw: add encode_json

rgw: add encode_json

dump() just dumps the internal content of an object, encode_json()
create the object inside its own section. Note that there are cases
where we don't want an object to be surrounded by a section, e.g.,
when an object represents an array. In such a case we'd need to
override the encode_json() for this object type.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>