Add kubeconfig variable substitution for kubelet and proxy.

There are checks for the kubeconfig for both kubelet and proxy which
the current kube-bench implementation does not check for properly.
kube-bench checks the wrong files.

This PR adds support for variable substitution for all the config file
types are that should be checked in the CIS benchmarks.

This PR also fixes a buggy in CIS 1.3.0 check 2.2.9, which checks for
ownership of the kubelet config file /var/lib/kubelet/config.yaml but
recommends changing ownership of kubelet kubeconfig file
/etc/kubernetes/kubelet.conf as remediation.