virtual_fuel: prepare class to allow multiple bridges

virtual_fuel: prepare class to allow multiple bridges

The VirtualFuel class has now two new methods:

    del_vm_nics: Deletes all interfaces from the VM.

    add_vm_nic: Adds a NIC to the VM, attached to the specified bridge.

The following method has been deleted:

    set_vm_nic: implemented with the two new methods

Apart from the deleted method, no functionality has been changed.

This is just a small but necessary step towards adding support for
configuring more than one NIC in the fuel VM.

Change-Id: I9f02c8163dfb9768510e78d5d5e77a0bb43306fb
Signed-off-by: Josep Puigdemont <josep.puigdemont@enea.com>