Multibackend availability support

Currently, all Cinder services on one node must be part of the same storage availability zone due to this option being in the DEFAULT config section. This made sense when only one backend was specified. But since adding multibackend support, and deprecating support for defining backends in the DEFAULT section, this puts an unnecessary restriction on backend service setup.

The API and scheduler service already need to be made available outside of a given availability zone/fault domain. Since the majority of backends are external devices, making the c-vol service just a control path like the api and scheduler service, it should be fine to include c-vol in with these others. The backends themselves can be in different fault domains. We should therefore allow defining per-backend availability zones.

For backward compatibility, the existing storage_availability_zone configuration setting will be kept in the DEFAULT section. If per-backend availability zones are not defined, this will work as it has up until now of placing all backends in that AZ.

There will be a new driver configuration option of backend_availability_zone that can be specified in the backend sections of the config. This will override the DEFAULT setting and place that backend in the different AZ.