(maint) Add option to select matrix mode
(maint) Add option to select matrix mode

Previously defaulted to sending a message for every cell of a matrix
job. Added option to select either ONLY_PARENT, ONLY_CONFIGURATIONS,
or BOTH.

puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder,puppetlabs/jenkins-job-builder