[FIXED JENKINS-20488] - Prevented possible NPE on nodes renaming/deletion

Resolves https://issues.jenkins-ci.org/browse/JENKINS-20488

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
[FIXED JENKINS-20488] - Missing nodes handling for builds

https://issues.jenkins-ci.org/browse/JENKINS-20488

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Use a proper helper to avoid node deletion issues.
Related to https://issues.jenkins-ci.org/browse/JENKINS-20488

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Minor code improvements according to FindBugs suggestions

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Removed recursive loop caused by fixing of https://issues.jenkins-ci.org/browse/JENKINS-20488

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
FindBugs cleanup

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>