Fix: initialize builder['plugins_info'] to None
Fix: initialize builder['plugins_info'] to None

Introduced by a7af4a4c100916f77456a8dabbc92a3f935bde62

config.validate() expects the presence of builder['plugins_info'].
Its value can either be null or a list.

Change-Id: Idc9f0be7b25cc254f2aea958d5c3bc8b3ccaac89
Signed-off-by: Jonathan Rajotte <e21d05e46a1af599d82244a4fa14885185afa8cf@efficios.com>

OSSystems/jenkins-job-builder,puppetlabs/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,wikimedia/integration-jenkins-job-builder,wikimedia/integration-jenkins-job-builder,wsoula/jenkins-job-builder,wsoula/jenkins-job-builder,puppetlabs/jenkins-job-builder,gforcada/jenkins-job-builder,gforcada/jenkins-job-builder,OSSystems/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,OSSystems/jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder,gforcada/jenkins-job-builder,OSSystems/jenkins-job-builder,gforcada/jenkins-job-builder,wsoula/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,wsoula/jenkins-job-builder,gforcada/jenkins-job-builder