[feat req] allow passing the Elasticsearch endpoint via environment vars

[tedder]

Currently it takes some nonstandard leaps to point the proxy to a host in an environment variable. The following is sort of 'pseudocode' for docker config.

doesn't work:

environment: ["ES_URL": "http://blah:9200"]
command: ["-endpoint","${ES_URL}"]

current workarounds:

environment: ["ES_URL": "http://blah:9200"]
command: ["sh", "-c", "aws-es-proxy -endpoint ${ES_URL}"]
command: "aws-es-proxy -endpoint ${ES_URL}"

it'd be nice to do:

environment: ["ES_URL": "http://blah:9200"]
command: [""]

It might be handy for other parameters, but this is the one that tends to vary the most- for instance, in ECS, I can look it up by storing it in ssm and then injecting that into ES_URL of the secrets section.

This is beyond my golang abilities, which is why I'm only putting it as a feature request, not a PR :/

[saravanan30erd]

@tedder Raised the PR for this.