Thanks. I'm utterly baffled right now:
[2019-04-11T13:06:50,465][TRACE][o.e.c.c.ClusterBootstrapService] [d-gp2-kyles-1] nodesMatchingRequirements=[], unsatisfiedRequirements=[d-gp2-kyles-1, d-gp2-kyles-2, d-gp2-kyles-3], bootstrapRequirements=[d-gp2-kyles-1, d-gp2-kyles-2, d-gp2-kyles-3]
[2019-04-11T13:06:50,470][INFO ][o.e.c.c.ClusterBootstrapService] [d-gp2-kyles-1] skipping cluster bootstrapping as local node does not match bootstrap requirements: [d-gp2-kyles-1, d-gp2-kyles-2, d-gp2-kyles-3]
These two messages are saying that the local node called d-gp2-kyles-1 doesn't match the d-gp2-kyles-1 in cluster.initial_master_nodes. As far as I can tell they're byte-for-byte equal, although one of them was typed directly into the config file and the other comes from the environment. Could you try setting node.name: d-gp2-kyles-1 explicitly in elasticsearch.yml instead of using ${HOSTNAME} so that they both come from the same place?
You can lose logger.org.elasticsearch.discovery: TRACE too, at least until we work out what the ClusterBootstrapService is struggling with.