Update clock sync guidance

[bdarnell]

We currently recommend that multi-cloud deployments use google's time service because we couldn't confirm that google and amazon smeared the leap second in compatible ways. (and google's time service is available in AWS but not vice versa). Google's doc has been updated to say that amazon's leap smear is compatible, so we can simplify this advice.

Now VMs deployed in GCE should use google's time service, and VMs deployed in AWS should use amazon's. Instances deployed anywhere else that need to join a cluster that includes nodes in GCE or AWS should use google's time service (or ensure that whatever time source they use is compatible with google's "standard smear").