dummy driver

Create dummy share driver for following testing purposes:
- Heavily test manila REST APIs without wasting real resources;
- Heavily test manila internal driver interfaces without wasting real resources.

It will allow us to run real-backend-based CI jobs with fewer amount of functional tests - only those that are backend-specific.

This share driver should behave in the same way as other driver as follows:
- Restrict usage of some share protocols
- Restrict usage of some access types
- Report specific set of capabilities
- Support both driver modes (DHSS=True/False)
- Support network plugins and other features