kubeadm config migrate uses AnyConfigFileAndDefaultsToInternal, which can
unmarshal config from file only if InitConfiguration or JoinConfiguration are
present. Even with that in mind, it can only return a singlie config object,
with InitConfiguration taking precendence over JoinConfiguration. Thus, the
following cases were not handled properly, while they were perfectly valid for
kubeadm init/join:

- ClusterConfiguration only file caused kubeadm config migrate to exit with
  error.
- Init + Join configurations in the same file caused Init + Cluster
  configuration to be produced (ignoring JoinConfiguration). The same is valid
  when the combo is Init + Cluster + Join configurations.
- Cluster + Join configuration ignores ClusterConfiguration and only
  JoinConfiguration gets migrated.

To fix this, the following is done:
- Introduce MigrateOldConfigFromFile which migrates old config from a file,
  while ensuring that all kubeadm originated input config kinds are taken care
  of. Add comprehensive unit tests for this.
- Replace the use of AnyConfigFileAndDefaultsToInternal in
  kubeadm config migrate with MigrateOldConfigFromFile.
- Remove the no longer used and error prone AnyConfigFileAndDefaultsToInternal.
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>