Mode selection for Manila drivers

Currently manila drivers have 2 "modes" (a term I'm willing to change):
* multi share server
* single share server

These modes are implicit based on how the driver is written and the administrator has no control. I would like to propose allowing administrators to explicity make this choice, and allowing drivers to accept/reject the choice, such that drivers can implement multiple modes if they choose to (implementing just 1 mode is still fine).

Furthermore, I'd like to introduce a 3rd mode which acts as a middle ground to the existing modes:
* single share server (old)
* flat network multi share server (NEW)
* segmented network multi share server (old)

The new mode would allow drivers to create share servers, but with greatly relaxed requirements on the underlying network configuration. This would enable deployers to take advantage of share servers without forcing them to adopt complex network topologies. In particular, it would simplify the process of starting with a small scale installation and scaling up, it would make proof-of-concept deployments much quicker, and it would simplify the lives of developers trying to write new drivers.