TypeError: __init__() missing 1 required positional argument: 'scheme'
Author: amirj | Pull request: no | State: closed

## Describe the bug
Based on [this tutorial](https://huggingface.co/docs/datasets/faiss_es#elasticsearch) the provided code should add Elasticsearch index but raised the following error, probably the new Elasticsearch version is not compatible though the tutorial doesn't provide any information about the supporting Elasticsearch version.

## Steps to reproduce the bug
```python
# Sample code to reproduce the bug
from datasets import load_dataset
squad = load_dataset('squad', split='validation')
squad.add_elasticsearch_index("context", host="localhost", port="9200")
```

## Expected results
[Creating an elastic index based on the provided tutorial](https://huggingface.co/docs/datasets/faiss_es#elasticsearch)

## Actual results
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-8fb51aa33961> in <module>
      1 from datasets import load_dataset
      2 squad = load_dataset('squad', split='validation')
----> 3 squad.add_elasticsearch_index("context", host="localhost", port="9200")

~/opt/anaconda3/lib/python3.8/site-packages/datasets/arrow_dataset.py in add_elasticsearch_index(self, column, index_name, host, port, es_client, es_index_name, es_index_config)
   3777         """
   3778         with self.formatted_as(type=None, columns=[column]):
-> 3779             super().add_elasticsearch_index(
   3780                 column=column,
   3781                 index_name=index_name,

~/opt/anaconda3/lib/python3.8/site-packages/datasets/search.py in add_elasticsearch_index(self, column, index_name, host, port, es_client, es_index_name, es_index_config)
    587         """
    588         index_name = index_name if index_name is not None else column
--> 589         es_index = ElasticSearchIndex(
    590             host=host, port=port, es_client=es_client, es_index_name=es_index_name, es_index_config=es_index_config
    591         )

~/opt/anaconda3/lib/python3.8/site-packages/datasets/search.py in __init__(self, host, port, es_client, es_index_name, es_index_config)
    123         from elasticsearch import Elasticsearch  # noqa: F811
    124 
--> 125         self.es_client = es_client if es_client is not None else Elasticsearch([{"host": host, "port": str(port)}])
    126         self.es_index_name = (
    127             es_index_name

~/opt/anaconda3/lib/python3.8/site-packages/elasticsearch/_sync/client/__init__.py in __init__(self, hosts, cloud_id, api_key, basic_auth, bearer_auth, opaque_id, headers, connections_per_node, http_compress, verify_certs, ca_certs, client_cert, client_key, ssl_assert_hostname, ssl_assert_fingerprint, ssl_version, ssl_context, ssl_show_warn, transport_class, request_timeout, node_class, node_pool_class, randomize_nodes_in_pool, node_selector_class, dead_node_backoff_factor, max_dead_node_backoff, serializer, serializers, default_mimetype, max_retries, retry_on_status, retry_on_timeout, sniff_on_start, sniff_before_requests, sniff_on_node_failure, sniff_timeout, min_delay_between_sniffing, sniffed_node_callback, meta_header, timeout, randomize_hosts, host_info_callback, sniffer_timeout, sniff_on_connection_fail, http_auth, maxsize, _transport)
    310 
    311         if _transport is None:
--> 312             node_configs = client_node_configs(
    313                 hosts,
    314                 cloud_id=cloud_id,

~/opt/anaconda3/lib/python3.8/site-packages/elasticsearch/_sync/client/utils.py in client_node_configs(hosts, cloud_id, **kwargs)
     99     else:
    100         assert hosts is not None
--> 101         node_configs = hosts_to_node_configs(hosts)
    102 
    103     # Remove all values which are 'DEFAULT' to avoid overwriting actual defaults.

~/opt/anaconda3/lib/python3.8/site-packages/elasticsearch/_sync/client/utils.py in hosts_to_node_configs(hosts)
    142 
    143         elif isinstance(host, Mapping):
--> 144             node_configs.append(host_mapping_to_node_config(host))
    145         else:
    146             raise ValueError(

~/opt/anaconda3/lib/python3.8/site-packages/elasticsearch/_sync/client/utils.py in host_mapping_to_node_config(host)
    209         options["path_prefix"] = options.pop("url_prefix")
    210 
--> 211     return NodeConfig(**options)  # type: ignore
    212 
    213 

TypeError: __init__() missing 1 required positional argument: 'scheme'
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.0.0
- Platform: Mac
- Python version: 3.8.0
- PyArrow version: 7.0.0
- ElaticSearch Info: 
{
  "name" : "byname",
  "cluster_name" : "elasticsearch_brew",
  "cluster_uuid" : "9xkjrltiQIG0J95ciWhqRA",
  "version" : {
    "number" : "7.10.2-SNAPSHOT",
    "build_flavor" : "oss",
    "build_type" : "tar",
    "build_hash" : "unknown",
    "build_date" : "2021-01-16T01:41:27.115673Z",
    "build_snapshot" : true,
    "lucene_version" : "8.7.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
  },
  "tagline" : "You Know, for Search"
}