Kubernetes should not have a CustomCommandLine.

As part of FLIP-73, all command line options are mapped to config options. Given this 1-to-1 mapping, the Kubernetes command line could simply forward the command line arguments to ConfigOptions directly, instead of introducing new command line options. In this case, the user is expected to simply write:

bin/run -e (or --executor) kubernetes-session-cluster -D kubernetes.container.image=MY_IMAGE ... 

and the CLI will parse the -e to figure out the correct ClusterClientFactory and ExecutorFactory and then forward to that the config options specified with -D. 

For this, we need to introduce a GenericCustomCommandLine that simply forward the specified parameters to the executors.