Reindex doesn't support _parent and _routing

[Mpdreamz]

(reported by @honzakral on the python client elastic/elasticsearch-py#140)

This is tied to elastic/elasticsearch#8068

In the meantime we need to:

enable pass fields=['_parent','_routing','_source'] to scan from reindex
change expand_action (or provide a different one used by reindex) to also look for the metadata in _fields of the hit dictionary

[Mpdreamz]

elastic/elasticsearch#8068 is resolved so we should be able to implement this