Make basic networking settings (ip/netmask/gateway) optional in GUI

We should add a flag in the configuration for writing basic network configuration (basically: /etc/network/interfaces).

This means we would make a flag, so the user does the network settings himself. I would recommand NOT letting the users set the hostname, we need it to often in different configuration files.
The flag should enable/disable writing of the following settings:
 * IP address
 * netmask
 * default gateway
 * Nameservers

everything else should still be set in the GUI. The DEFAULT settings should be to NOT overwrite existing system network settings, but to encourage the user to use the GUI later on.