refactor model: Create a separated model for interface and network resources

To avoid duplicating code in model and mockmodel, the code to interface
resource was added to model_/interfaces.py and the specific code for
each backend was added to backend/interfaces.py and
mockbackend/intarfaces.py

And the code related to network resource was added to model_/networks.py and
the specific code for each backend (libvirt or mock) was added to
backend/networks.py and mockbackend/networks.py

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
refactor model: Create a separated model for template resource

To avoid duplicating code in model and mockmodel, the code related to
template resource was added to model_/templates.py and
the specific code for each backend (libvirt or mock) was added to
backend/templates.py and mockbackend/templates.py

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
refactor model: Create a separated model for vm resource

To avoid duplicating code in model and mockmodel, the code related to vm
resource was added to model_/vms.py and the specific code for each backend
(libvirt or mock) was added to backend/vms.py and mockbackend/vms.py

Signed-off-by: Aline Manera <alinefm@br.ibm.com>