[FIXED JENKINS-20560] - Custom-tools should not override global paths inside nested launchers

I've also refactored retrieval of paths list.
Related to: https://issues.jenkins-ci.org/browse/JENKINS-20560

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
[JENKINS-20560] - Don't try to override empty paths
Caused by the initial fix of https://issues.jenkins-ci.org/browse/JENKINS-20560

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Merge pull request #13 from synopsys-arc-oss/JENKINS_20560_FIX

[FIXED JENKINS-20560] - Custom-tools should not override global paths in...