Configure ElasticSearch URL prefix using environment variable

[AndreLouisCaron]

I run elastalert inside a container against multiple environments and the value I need to put in es_url_prefix changes depending on the environment.

Currently, I need to write a new config.yaml file for each environment and bind-mount it into the container, which is pretty tedious. I can already configure es_host, es_port and use_ssl using environment variables, so I though doing the same for es_url_prefix would be neat.

Let me know what you think :-)

[AndreLouisCaron]

@Qmando I don't mean to be pushy (I assume you are very busy with other matters), but I'm still hoping to get this minor change merged as it solves a workflow issue for me. Any chance you can take a minute to look at this?

Thanks!