Fix "route add default dev <iface>" behaviour

Previous behaviour was to set route's network mask as 255.255.255.255
Setting a destination network with netmask /32 blocks the connections
just to the specified host. 0.0.0.0/32 defines the exact IPv4 address
0.0.0.0 and not the whole IPv4 hosts range.
This patch allows traffic to any network, setting a 0.0.0.0/0 route.

Change-Id: I1665f2fac52526337bb2c48e3b09564d9da448e7