iproute: change the default scope for addr

The default scope for IPv4 addr was 0xfe (host), but other
utilities use 0x0 (global). This makes impossible to add
secondary ip address, when the primary was added by iproute2,
e.g. So use the scope 0x0 here, and make it customizable.

Bug-Url: https://github.com/svinota/pyroute2/issues/27