Allow to configure the Elasticsearch connection in environment variables

[josegar74]

This pull request allows to configure the Elasticsearch connection using environment variables.

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[davidblasby]

@josegar74 - I've noticed some inconsistencies in the configuration (nothing to do with this PR).

By this I mean;

1. If I set es.host (vi the EnvVars), then it doesn't really really get used. It will be replaced with es.url - which will be localhost (defined earlier).
2. the EsRestClient partially used es.url and partially es.* vars.
   • RestClientBuilder builder = RestClient.builder(new HttpHost(serverHost, Integer.parseInt(serverPort), serverProtocol));

• this is using the defined es.port, es.host, etc.. NOT THE es.url
• the next line if (serverUrl.startsWith("https://")) -- this is using es.url INSTEAD of es.protocol

So, you must:

1. define all the es.* variables
2. have es.url consistent with the set variables.

I'm not quite sure what will happen if this is run in a WAR - I don't think es.* will be set as per the pom.xml.

I'm not quite sure how to fix this so its consistent and doesn't affect current installations.
I think we need to ignore es.url - its not really being used anywhere and the main config comes from the es.* variables.

[davidblasby]

@josegar74 - I created this PR (to this PR) that fixes those inconsistencies.

However, you can probably just merge this with just a doc change: Note that the individual components AND es.url MUST BE consistent.