Allow non tls client nodes

[runningman84]

It would be cool to have an option to run client nodes without tls. There is software like graylog which does not like the self signed certs created by the operator.

Connected to: https://github.com/upmc-enterprises/elasticsearch-operator/issues/5 https://github.com/Graylog2/graylog2-server/issues/4153

[stevesloka]

This is a feature which we've documented (#5). Also, @gianrubio has a PR in progress on this, not sure what else is left (#64)

Certs that are auto-generated are also exposed as secrets in the same namespace. You'd have to spin up the elastic cluster first, but could have Graylog pull those in maybe as an alternative.

[gianrubio]

#161 fixed this issue