(FACT-624) Add structured uptime fact

This commit adds a new 'system_uptime' fact, which returns a map including
keys for seconds, hours, days and a human readable uptime.

Also, the existing uptime facts were updated to take their values from this new 
structured fact, rather than resolving them individually.
Aha, I'll fix that. It actually was enforcing it with one too few spaces, but apparently not for one too many.