Feature: Support for proxy parameter in ElasticSearch client

[sebastien]

Currently, there is no way to specify HTTP proxy urls for the ElasticSearch client to use when performing requests. Although issue #275 is marked as closed, there is no evidence of a solution implemented and supported by the client. We have tried to apply the recommended solutions, but none of the following have worked:

• Using node_class="requests" or node_class=RequestsHttpNode along with HTTP_PROXY and HTTPS_PROXY environment variables set
• Using a custom subclass of RequestsHttpNode to set the proxies at object construction

In both cases, requests were not performed through the given proxy URL.

We propose to the addition of a proxy parameter (or a proxy_http, proxy_https parameter set) to support the definition of a proxy to be used by a transport backend.