Ever since v1alpha3, InitConfiguration is containing ClusterConfiguration
embedded in it. This was done to mimic the internal InitConfiguration, which in
turn is used throughout the kubeadm code base as if it is the old
MasterConfiguration of v1alpha2.

This, however, is confusing to users who vendor in kubeadm as the embedded
ClusterConfiguration inside InitConfiguration is not marshalled to YAML.
For this to happen, special care must be taken for the ClusterConfiguration
field to marshalled separately.

Thus, to make things smooth for users and to reduce third party exposure to
technical debt, this change removes ClusterConfiguration embedding from
InitConfiguration.
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>