Currently kubeadm supports a couple of configuration versions - v1alpha3 and
v1beta1. The former is deprecated, but still supported.

To discourage users from using it and to speedup conversion to newer versions,
we disable the loading of deprecated configurations by all kubeadm
sub-commands, but "kubeadm config migrate".

v1alpha3 is still present and supported at source level, but cannot be used
directly with kubeadm and some of its internal APIs.

The added benefit to this is, that users won't need to lookup for an old
kubeadm binary after upgrade, just because they were stuck with a deprecated
config version for too long.

To achieve this, the following was done:

- ValidateSupportedVersion now has an allowDeprecated boolean parameter, that
  controls if the function should return an error upon detecting deprecated
  config version. Currently the only deprecated version is v1alpha3.

- ValidateSupportedVersion is made package private, because it's not used
  outside of the package anyway.

- BytesToInitConfiguration and LoadJoinConfigurationFromFile are modified to
  disallow loading of deprecated kubeadm config versions. An error message,
  that points users to kubeadm config migrate is returned.

- MigrateOldConfig is still allowed to load deprecated kubeadm config versions.

- A bunch of tests were fixed to not expect success if v1alpha3 config is
  supplied.
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>