Changing the network interfaces listing

- InterfacesModel now lists all available interfaces in the host,
sorted by interface name.

- InterfaceModel adds a new attribute to the lookup method called
'editable', which points whether the interface can be configured
by the user. The UI should take this parameter into consideration
to disable editing in these interfaces.

- The backend will raise an error if an attempt to edit a
non-editable interface is made by the frontend.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>