Optionally use generated api_key auth to ES

This commit adds a optional client arg, use_api_key, which will generate
and use an api key for every client and async client created by
Rally. It uses an initial call via the existing auth scheme to ES to
generate the key, and then creates a new client using the api_key for
general use by Rally.

The async client also does this, and temporarily creates a sync client
to generate the key, and then uses it when creating an async client.

Closes elastic#1067

esrally/client.py

@@ -129,7 +129,17 @@ def _is_set(self, client_opts, k):
 
     def create(self):
         import elasticsearch
-        return elasticsearch.Elasticsearch(hosts=self.hosts, ssl_context=self.ssl_context, **self.client_options)
+        instance = elasticsearch.Elasticsearch(hosts=self.hosts, ssl_context=self.ssl_context, **self.client_options)
+        if self._is_set(self.client_options, "use_api_key"):
+            # an api_key is generated per client, which happens after an initial handshake using an existing auth scheme
+            # once this key is generated, a new client is created using the api_key client option and http_auth is removed
+            # the reason the options were not removed in place was because of the logic to coalesce the key tuple into a
+            # header that the client could use.
+            api_key_response = instance.security.create_api_key({"name": "rally-api-key"})
+            self.client_options.pop("http_auth")
+            self.client_options["api_key"] = (api_key_response["id"], api_key_response["api_key"])
+            instance = elasticsearch.Elasticsearch(hosts=self.hosts, ssl_context=self.ssl_context, **self.client_options)
+        return instance
 
     def create_async(self):
         import elasticsearch
@@ -139,6 +149,10 @@ def create_async(self):
 
         from elasticsearch.serializer import JSONSerializer
 
+        if self._is_set(self.client_options, "use_api_key"):
+            # Temporarily create a non async version of the client to generate an api_key and put it into client_options
+            self.create()
+
         class LazyJSONSerializer(JSONSerializer):
             def loads(self, s):
                 meta = RallyAsyncElasticsearch.request_context.get()