watchcat: use logical network to fix restart_interface

watchcat: use logical network to fix restart_interface

Watchcat was failing to restart layer-3 interfaces when in mode
'restart_iface'. The previously attempted fix made the situation
worse in that it resulted in layer 2 interfaces also failing to
start.

This was because we are passed the interface name (e.g. eth0,
l2p0, or br-lan), but ifup needs the logical network (e.g. 'lan'
which corresponds to the network device).

Update to use find_config from /lib/network/config.sh to find the
logical network from the interface name, and use ifup on the
logical network to restart the underlying interface(s) associated
with the logical network.

Signed-off-by: Daniel F. Dickinson <redacted>