Fuel client rafactoring part 2

It's a continuation of https://blueprints.launchpad.net/fuel/+spec/re-thinking-fuel-client from 6.1 where fuelclient was moved from nailgun to seperate repo.

Now more improvements are required:
1. Finish moving fuel commands to fuel2 (new interface with updated and refactored commands, new functionalities should be add only to fuel2, it will replace fuel command when all commands will be migrated)
- Rewrite objects and interfaces
- Finish rewriting client to use cliff 10% is done

2. Make APIClient/fuelclient usable as a library.
- add library-specific code which client will use
- remove singleton APIClient
- remove “sys.exit”

3. Improve fuelclient testing
- allow to test fuelclient against specific nailgun version
- add more integration tests
- test fuelclient for each change in nailgun

4. Create documentation for fuelclient (both dev and user oriented)
- describe how to add new commands to fuel2

5. Notify about new commands being added only to fuel2