Add JSON responses to Network and Subnet models

Signed-off-by: Tomas Sedovic <tomas@sedovic.cz>
Fix Network and Subnet create routes for JSON

Creating a network or subnet whilst using "Accept: application/json"
resulted in an exception because of the missing `xml_to_json` method in
the routes for JSON responses.

Signed-off-by: Tomas Sedovic <tomas@sedovic.cz>
JSON support for network_interface model

Signed-off-by: Tomas Sedovic <tomas@sedovic.cz>