Are you sure you’re running 15.10? AFAIK there is either 15.04 or Ubuntu Core 16.
That /etc/network/interfaces is read-only lets me think you’re on the later one. On Ubuntu Core 16 you can’t configure the network through /etc/network/interface anymore. If NetworkManager is installed on your system you have to configure the network through
nmcli utility. If NetworkManager is not installed you can configure the network through netplan. Please see the docs here for details on the configuration through NetworkManager and here for details on netplan.
If you have NetworkManager installed, please don’t attempt to use netplan. Mixing NetworkManager and netplan is not well supported yet.