Link to the correct ES Python client docs. (#1554)

We'll now programmatically set the ES client version in the docs build.

docs/command_line_reference.rst

@@ -663,7 +663,7 @@ By default, the command line reporter will print the results only on standard ou
 
 With this option you can customize Rally's internal Elasticsearch client.
 
-It accepts a list of comma-separated key-value pairs. The key-value pairs have to be delimited by a colon. These options are passed directly to the Elasticsearch Python client API. See `their documentation on a list of supported options <http://elasticsearch-py.readthedocs.io/en/master/api.html#elasticsearch.Elasticsearch>`_.
+It accepts a list of comma-separated key-value pairs. The key-value pairs have to be delimited by a colon. These options are passed directly to the Elasticsearch Python client API. See `their documentation on a list of supported options <https://elasticsearch-py.readthedocs.io/en/v{ES_CLIENT_VER}/api.html#elasticsearch>`_.
 
 We support the following data types:
 

docs/conf.py

@@ -54,8 +54,15 @@ def read_min_python_version():
         raise ConfigError(f"Failed building docs as required key [{e}] couldn't be found in the file [{CI_VARS}].")
 
 
+def get_es_client_version():
+    import elasticsearch
+
+    return ".".join(map(str, elasticsearch.__version__))
+
+
 GLOBAL_SUBSTITUTIONS = {
     "{MIN_PY_VER}": read_min_python_version(),
+    "{ES_CLIENT_VER}": get_es_client_version(),
 }
 
 
@@ -82,6 +89,7 @@ def read_min_es_version():
 .. |year| replace:: {year}
 .. |MIN_PY_VER| replace:: {read_min_python_version()}
 .. |min_es_version| replace:: {read_min_es_version()}
+.. |ES_CLIENT_VER| replace:: {get_es_client_version()}
 """
 
 # General information about the project.