Support HTTP proxies in eland_import_hub_model (#688)

* Document TLS/SSL options for import script

* Mention --help option

* Add HTTP proxy support

* Mention HTTP_PROXY too

---------

Co-authored-by: David Kyle <david.kyle@elastic.co>

docs/guide/machine-learning.asciidoc

@@ -154,6 +154,23 @@ underscores `__`.
 
 --
 
+[discrete]
+[[ml-nlp-pytorch-proxy]]
+==== Connect to Elasticsearch through a proxy
+
+Behind the scenes, Eland uses the `requests` Python library, which
+https://requests.readthedocs.io/en/latest/user/advanced/#proxies[allows configuring
+proxies through an environment variable]. For example, to use an HTTP proxy to connect to
+an HTTPS Elasticsearch cluster, you need to set the `HTTPS_PROXY` environment variable
+when invoking Eland:
+
+[source,bash]
+--------------------------------------------------
+HTTPS_PROXY=http://proxy-host:proxy-port eland_import_hub_model ...
+--------------------------------------------------
+
+If you disabled security on your Elasticsearch cluster, you should use `HTTP_PROXY`
+instead.
 
 [discrete]
 [[ml-nlp-pytorch-auth]]

eland/cli/eland_import_hub_model.py

@@ -164,6 +164,7 @@ def get_es_client(cli_args, logger):
             "request_timeout": 300,
             "verify_certs": cli_args.insecure,
             "ca_certs": cli_args.ca_certs,
+            "node_class": "requests",
         }
 
         # Deployment location