[openFATE 325606] Support picking single multibuild containers

Feature changed by: Adrian Schröter (adrianSuSE) Feature #325606, revision 2 Title: Support picking single multibuild containers - Buildservice: New + Buildservice: Evaluation by engineering manager Milestone: 2.10 Priority Requester: Mandatory + Projectmanager: Mandatory Requested by: Adrian Schröter (adriansuse) Requested by: Dominique Leuenberger (dimstar) Partner organization: openSUSE.org Description: Factory Ring setup needs a way to pick single multibuild containers only. Current proposal is to a _link file with a special flavor element, which can be used to filter from defined _multibuild flavors. -- openSUSE Feature: https://features.opensuse.org/325606