Unit tests for `kubeadm upgrade diff`

[liztio]

Some suggestions from @neolit123:

• non existing config path in flags.parent.cfgPath
• flags.newK8sVersionStr == "", but no version arg is provided
• bad version format in flags.newK8sVersionStr to make ParseSemantic() fail
• make kubeadmutil.MarshalToYaml fail somehow.
• fail ioutil.ReadFile(path) by providing bad 'path'

[neolit123]

^ submitted PR for the UTs.