RMD to support multiple SG Managers

As we evolve as a project, being able to innovate at a fast pace is very important, especially as far as the interaction with other Openstack Projects is concerned.
More specifically, the interaction with Neutron's Security Groups is a very important piece of our logic, and we should be capable of easily switch between different ways of mapping GBP constructs to Security Groups in order to improve efficiency and capabilities, by still keeping a way to provide backward compatibility for existing deployments.

For these reasons, this blueprint aims to build a plugin architecture for Security Group management in the RMD.
The first implementations will include:

- remote_subnet_manager, which keeps the same logic we have used so far for SG mapping;

- remote_group_manager, a new SG manager which uses remote-SG logic for easier rule management, and also creates multiple SGs depending on their scope in order to provide shareability of PRSs.