Open Stack

I think basing the plugins against master is the best solution since
the plugin in development will want to work with the matching horizon
release. Finding issues sooner and hopefully one at a time is a lot
easier to address than a bundle of them at the end of a release cycle.
Hopefully, Horizon doesn't break plugins on a regular basis. If we
are, then we are not being focused enough around supporting plugins by
maintaining compatibility and focused too much around what's easiest
for Horizon in the development process. Horizon is no longer at the
top of the stack, we should develop with that awareness. Of course,
breaking changes will happen, but they should be limited and isolated.

David