Allow the user to pass custom cert validity period with
ClusterConfiguration.CertificateValidityPeriod and
CACertificateValidityPeriod.
The defaults remain 1 year for regular cert and 10 years for CA.
Show warnings if the provided values are more than the defaults.
Additional changes:
- In "certs show-expiration" use HumanDuration() to print
more detailed durations instead of ShortHumanDuration().
- Add a new kubeadm util GetStartTime() which can be used
to consistently get a UTC time for tasks like writing certs
and unit tests.
- Update unit tests to validate the new customizable NotAfter.
