The root command will run node checks and if possible master checks.
I've also added some Makefile targets to improve local testing and improve the documentation.

Made all the warnings to be logged and not printed, so when using the json flag the output will be only in json format.

fix #217

The CIS benchmark check for node checks 2 config files for kubelet:
  - kubelet config file (kubelet.conf)
  - kubelet systemd unitfile (10-kubeadm.conf)

The getServiceFiles function gets candidates for kubelet systemd
unitfile and returns valid untifiles.

If people are trying to use the Docker image to check their cluster, there's a
big likelyhood of them hitting the error message saying that either `kubectl`
or `kubelet` need to be found in order for `kube-bench` to be able to determine
the Kubernetes version in use.

This adds a tip that the version can be specified manually with the `--version`
flag which is a lot easier than having to make a new Docker image with the
right version of `kubelet`/`kubectl` in order for `kube-bench` to work.

This change unifies all config files, podspecs and unitfiles under
a single component configuration key; `config`.

improve error messages.

The server version is used to load the correct benchmark check
to run against the Kubernetes cluster.

Reorder YAML to make a bit more sense. Allow for optional components, and a config file that we don’t think exists. 

printlnWarn: prints warning with a newline.
sprintWarn: returns an optionally contextualized warning string.