Part of what was decided at summit is devstack needs to return to a
more opinionated stance, the following removes support for non
RabbitMQ messaging. RabbitMQ is used by over 95% of our community
(statistically all of it), so it's a pretty clear line to draw that
this shouldn't be in tree.
iniset_rpc_backend will be our stable hook for other projects that
want to implement this out of tree. The burden on creating those out
of tree plugins will be on those that wish to support those
alternative stacks.
Change-Id: I8073a895c03ec927a2598eff6c2f01e5c82606fc
37eca48970