Document practical use case for variable defaults
Document practical use case for variable defaults

Took inspiration from my StackOverflow reply here
https://stackoverflow.com/questions/50177308 to document a practical
use of setting variable defaults and inheritenence.

Change-Id: I45934341d3ff3fbf4ff711a435924197d2f12712
Signed-off-by: Thanh Ha <ddef40ffcf330e0c1e91b15bb1b0d4953f1680d8@linux.com>

wsoula/jenkins-job-builder,wsoula/jenkins-job-builder,gforcada/jenkins-job-builder,gforcada/jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder,wsoula/jenkins-job-builder,gforcada/jenkins-job-builder,wsoula/jenkins-job-builder,gforcada/jenkins-job-builder,gforcada/jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder