Also fixes JENKINS-25498

Node monitor refactorization

change BuildStability message into more appropriate one

[FIXED JENKINS-25788] RunMap.remove was always returning false, even when the Run was in fact removed.

Conflicts:
	changelog.html

Tested on a windows box to automaticaly install 1.4, 1.5, 1.6, 1.7 and 1.8 JDKs

[FIXED JENKINS-23797] Produce human readable output in DiskSpaceMonitorDescriptor.DiskSpace.toString()

[FIXED JENKINS-23966] Merging #1474.

Cf.: https://github.com/jenkinsci/jenkins/commit/d29a90150c839855d4ff0c084c3cd838b48fab5c#commitcomment-8845400 (pending #806).