Cycle-trailing deliverables are regular cycle-following deliverables,
using RCs or not not using RCs -- they just have a different deadline.
Rather than using a release model, those deadline variants are better
described using deliverable types, in much the same way 'library'
deliverables have a specific deadline too.
This simplifies the list of models significantly, and allows to have
proposer validation of trailing deliverables that use RCs or not use
RCs.
For compatibility in old branches, setting 'cycle-trailing' is still
supported, it will just overload the type to 'trailing' if specified.
Change-Id: Ifce88ef3e5dd406f45f25214699f16e736ad5377
8a9f755cb9