Include TANGO_HOST in dump output

It seems like a good idea to include a tango_host field in the output of tango2json. That way it would be unambiguous where the data comes from. I think we could just add it as a top level field, it should not matter to any use case I think. E.g.

{
    "tango_host": "b-v-veritas-csdb-0:10000",
    "servers": {
...

Then we could also consider checking that the tango host is correct, in json2tango. Probably as an optional feature, to not break any current use.