Spark Charm -> Spark 2.4

@kwmonroe @hecklerchris @magicaltrout

I’m thinking what this means for juju deployed spark is that we can easily build a spark k8s charm by using base images e.g. gcr.io/spark-operator/spark:v{2.2x, 2.3.x,2.4.x} or our own custom images built FROM the desired upstream ones.

Possibly this is a way to go about the BigTop Vs. Spark 2.4 issue going forward.