kubeadm: generalise CreateOrUpdate etc.

This uses generics to generalise the various CreateOrUpdate,
CreateOrRetain etc. functions. Where appropriate, the context is added
as an initial argument to the new functions.

ConfigMapMutator isn't used anywhere else, so it's dropped in favour
of the private objectMutator added in this commit.

Signed-off-by: Stephen Kitt <[email protected]>

Author: skitt