Omit configuration for a local clock as time source if running on a VM.

Quoting http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427
'It is also important not to use the local clock as a time source, often referred to as the Undisciplined Local Clock.'
Merge pull request #49 from alexjfisher/feature/master/disable_local_clock_for_vms

Omit configuration for a local clock as time source if running on a VM.