Should be able to configure a fallback or priority of sources

[mwtrigg]

The priority of the various possible time sources should be configurable.

One should be able to configure a preference or priority of time sources, such that if PTP fails or is unavailable, instead of reverting to the system clock and drifting, the machine can fallback to a different PTP master, or a different PTP domain, or an array of NTP sources, or the domain hierarchy.

The service should make periodic checks to flip back to its preferred time configuration.