Feature: add ability to specify Akka.Cluster SplitBrainResolver strategy via environment variable

[Aaronontheweb]

We should add support for running our pre-built Lighthouse Docker images with an Akka.Cluster SplitBrainResolver - currently it's not possible to enable one of these without building a custom Docker image, and it's considered to be a best Akka.Cluster practice to run with a consistent split brain configuration available on all cluster nodes.

Therefore, we should add an ability to configure this via an environment variable:

SPLIT_BRAIN_STRATEGY=none|keep-majority|keep-oldest|static-quorum|keep-referee

Some of these strategies, such as static quorum and keep-referee, are going to require additional environment variables to support their mandatory parameters (quorum size and referee address, respectively.)

We should add these to the next release of Lighthouse and to the Lighthouse documentation.

The default value in Lighthouse should be keep-majority

[ghost]

As shown in the image below, the docker image uses the keep-majority strategy by default.