Configuration without new docker image

[slitsche]

We should enable passing a map of configuration options to the docker image. This will remove the need to build a new docker image (which would require a rolling update) in case we need a new configuration option.

[a1exsh]

We should also store the generated configuration file(s) outside of the docker image, on the data volume. This will allow to change configuration w/o removing the existing docker image.

[a1exsh]

A sad surprise is that Debian-packaged cassandra.in.sh sets CASSANDRA_CONF=/etc/cassandra w/o an option to override it. So it looks like we'll have to supply an edited version of this file as well, in addition to a custom cassandra-env.sh that we already provide...