Rethink default value of kubernetes.container.image

Currently, the default value of the configuration option kubernetes.container.image is set to flink:latest. This has the effect that we will always start the latest Flink version independent of which version we are using to start the Kubernetes cluster.

I am a bit unsure whether this is a good or not so good behaviour and what the user would actually expect to happen. Hence, this issue is more to discuss the pros and cons about the current default value and whether we should set it to a fixed version.

One problem I could see is that we are providing some files from the local Flink installation which might be incompatible with flink:latest. E.g. at the moment we think about upgrading Flink's log4j dependency to log4j2. Log4j2 requires a different configuration file which will most likely replace the existing log4j.properties file in Flink's binary distribution. If we now start a K8s session cluster with an older version where we still have the old log4j.properties file, then the logging might not work with a version where we are using log4j2. Hence, it might be safer to fix the image version or at least do not allow that it uses a different major version and only the latest bug fix version.

If we should decide to fix the version, then we would need to update the default value with every major release. If this is the case, then the release guide needs to be updated.