GitLab

Replace use of global RAPI client, with the cached client from the
VirtualMachine or Backend model.

Dispatch network actions to all backends.

- Create Backend model holding info about credentials, status and resources
- Add Backend as foreign key to VirtualMachine
- Create a cache of Backend clients (RAPI) for better performance

Rename backend_id field of VirtualMachine model to avoid conflict between the
backend that hosts the VM and the id that the VM will have on the backend.

Added log folder option / Now delete-stale deletes also stale networks / Fixed failed testcases output not "None"

- bump snf-common required version
- replace ampqlib to puka