Enable automatic leader election by default for new installs

## What?

Builds off of the work from https://github.com/Graylog2/graylog2-server/pull/11373 .

Enable automatic leader election by default for new installs

`leader_election_mode=automatic`

Currently in https://github.com/Graylog2/graylog2-server/blob/master/misc/graylog.conf we have `is_leader = true`.

It may also be useful to allow nodes participating in leader_election_mode=automatic to not be eligible (although it is possible that omitting `leader_election_mode` and setting `is_leader = false` accomplishes this.

## Why?

Currently, and in line with current documentation, the leader role is explicitly configured for graylog clusters. This creates a single point of failure for graylog clusters. Allowing automatic leader election will simplify the graylog cluster creation process and can allow clusters to, by default, have better resiliency if the master node is offline or has a failure.