Enables Meta-Build System rebuilds of downstream projects after autolanding the current project.
This includes: - A new mbs-rebuild template to define the parent rebuild jobs, the autolanding jobs are reused as the builder jobs. - Updates to the mbs templates to reduce the number of jenkins parameters created. - A new archive directory structure to support automatic archive naming from the stack name in the future. - New MBS support for the head/mir stack - Updates to the oif and unity stacks to use this new method. Fixes: https://bugs.launchpad.net/bugs/1155676.