Warn only once for masked definitions
Warn only once for masked definitions

Currently JJB warns over 700 times when checking project-config with:
  WARNING:jenkins_jobs.registry:You have a macro ('tox') defined for
  'builder' component type that is masking an inbuilt definition

Warn only once for this.

Change-Id: I010765269c499320fc9a84b9493d22949080e9a0

phinexus/jenkins-job-builder,klmitch/jenkins-job-builder,lukas-bednar/jenkins-job-builder,dataxu/jenkins-job-builder,lukas-bednar/jenkins-job-builder,rsig/jenkins-job-builder,phinexus/jenkins-job-builder,dataxu/jenkins-job-builder,dataxu/jenkins-job-builder,tamac-io/jenkins-job-builder,rsig/jenkins-job-builder,twc-openstack/jenkins-job-builder,gforcada/jenkins-job-builder,onshape/jenkins-job-builder,wsoula/jenkins-job-builder,gforcada/jenkins-job-builder,tamac-io/jenkins-job-builder,gforcada/jenkins-job-builder,twc-openstack/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,lukas-bednar/jenkins-job-builder,mjeanson/jenkins-job-builder,onshape/jenkins-job-builder,onshape/jenkins-job-builder,david-caro/jenkins-job-builder,rsig/jenkins-job-builder,david-caro/jenkins-job-builder,OSSystems/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,OSSystems/jenkins-job-builder,wsoula/jenkins-job-builder,phinexus/jenkins-job-builder,joostvdg/jenkins-job-builder,dataxu/jenkins-job-builder,twc-openstack/jenkins-job-builder,joostvdg/jenkins-job-builder,mjeanson/jenkins-job-builder,wsoula/jenkins-job-builder,lukas-bednar/jenkins-job-builder,joostvdg/jenkins-job-builder,klmitch/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,david-caro/jenkins-job-builder,wsoula/jenkins-job-builder,twc-openstack/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,klmitch/jenkins-job-builder,tamac-io/jenkins-job-builder,joostvdg/jenkins-job-builder,mjeanson/jenkins-job-builder,wikimedia/integration-jenkins-job-builder,OSSystems/jenkins-job-builder,mjeanson/jenkins-job-builder,david-caro/jenkins-job-builder,tamac-io/jenkins-job-builder,puppetlabs/jenkins-job-builder,onshape/jenkins-job-builder,rsig/jenkins-job-builder,puppetlabs/jenkins-job-builder,phinexus/jenkins-job-builder,gforcada/jenkins-job-builder,klmitch/jenkins-job-builder,gforcada/jenkins-job-builder,phinexus/jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder,OSSystems/jenkins-job-builder