kubeadm: updated cmd tests as discussed in SIG

After today's SIG meeting, it was discussed how to proceed with these
types of test-cmd tests.  They will live in kubeamd/test/cmd and will
provide a flag that will allow you to skip them (--kubeadm-cmd-skip) and
by default will fail if kubeadm binary is not present

hack/verify-flags/known-flags.txt

@@ -325,6 +325,7 @@ kube-master
 kube-master-url
 kube-reserved
 kubeadm-path
+kubeadm-cmd-skip
 kubecfg-file
 kubectl-path
 kubelet-address