Blueprint-nova-cells-loadbalancing

Rationale
In some situations the nova-cell process can become a bottle neck and not consume messages from the RPC system fast enough.

Goal
This blueprint sets out to enable multiple 'nova-cell' consumers from a single RPC queue.

It should work all scenarios, for example:
 * Global cell
 * nova-api instances put messages in the queue, multiple nova-cell instances push those messages down to child cells


 * Child cell
 * incoming command messages come from a parent cell into our RPC queue, multiple 'nova-cell' workers consume and process these messages.