Reduce the HTTP interations with Elasticsearch nodes during configuration

[andsel]

The healthcheck! method interacts with Elasticsearch to grant that connections in the pool are good, plus it does a preflight check during register phase.

This led to 4 HTTP calls to Elasticsearch, where some could be collapsed:

1. HEAD request
2. GET '/' for elasticsearch? on register_phase
3. GET '/' to get elasticsearch version
4. GET '/_license' check

Which could be reduced to only 2 calls:

1. GET '/' for healthcheck (and always confirm it's ES)
2. GET '/_license'