Read kubernetes version from environment (#390)

* Read kubernetes version from environment

Set kubernetes version to the value of the environment variable `KUBE_BENCH_VERSION` if it is defined and the flag `--version` is not specified on the kube-bench command line.

The command line flag `--version` takes precedence of the environment variable `KUBE_BENCH_VERSION` if both are defined.

* Add info about KUBE_BENCH_VERSION to README