Move the defaulting of the BootstrapToken type inside the
bootstraptoken/v1 package. This prevents an error where
codegen complains that a defaulter for the type exists in both
the kubeadm v1beta3 and v1beta4.
Adapt kubeadm code to use the defaulter function and constants
that were moved to bootstraptoken/v1.
NOTE: technically this is a breaking change for direct users of
v1beta3/SetDefaults_BootstrapToken().
