In v1alpha3's, control plane component config options were nested directly into
the ClusterConfiguration structure. This is cluttering the config structure and
makes it hard to maintain. Therefore the control plane config options must be
separated into different substructures in order to graduate the format to beta.

This change does the following:

- Introduces a new structure called ControlPlaneComponent, that contains fields
  common to all control plane component types. These are currently extra args
  and extra volumes.

- Introduce a new structure called APIServer that contains
  ControlPlaneComponent and APIServerCertSANs field (from ClusterConfiguration)

- Replace all API Server, Scheduler and Controller Manager options in
  ClusterConfiguration with APIServer, ControllerManager and Scheduler fields
  of APIServer and ControlPlaneComponent types.
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>