METRON-1823 Refactor Elasticsearch Configuration Settings

[nickwallen]

This refactors the various ElasticsearchWriter settings so they are all defined in a single place as constants. There is no functional change. This is simply a refactor.

This is the refactor I was looking at doing for #1218, but decided to keep it separate to simplify the review process.

Testing

1. Spin-up the development environment.

2. Ensure alerts are visible in the Alerts UI.

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[justinleet]

@mmiklavc You were looking at migration of some of the ES stuff, right? Is this something worth pulling in with your changes, or should we just hold off for now?

[mmiklavc]

@justinleet correct, I have a number of changes that will overlap with or replace this in the ES client upgrade/migration. In particular the utils class changes.