[JENKINS-7776] Show decimals when disk space monitored as less than 1 GB.

The logic relied on the monitor being triggered to show more decimals.
The monitor threshold can be set to something other than 1 GB, and therefore didn't always work as intended.
[JENKINS-7776] Show human readable units when Monitored space is low.

Revert the logic in DiskSpaceMonitor to be correct again.
Use Functions.humanReadableByteSize() to show useful values in Monitors.
[FIXED JENKINS-7776] Merge pull request #1039 from ChrisA89/moreHelpfulDiskSpaceMonitor
Noting #1039