BusyBox ifup translates 'gateway X' into 'ip route add default via X'
and aborts the whole ifup run with RC=1 when that command fails with
'File exists' — which always happens on first boot because the docker
management bridge has already installed its own default via eth0. As a
result the 'post-up ip route replace default' was never executed and
the host kept the management default.
Remove the 'gateway' line so ifup only runs the idempotent 'post-up ip
route replace default via <fabric-gw>' and the fabric default wins.
Make hosts/campus-host{1,2}_interfaces the source of truth for the
campus host IP and default route, and have clab simply invoke 'ifup
eth1' at container start to apply it. Previously the bound interfaces
file was unused and the real config lived in the YAML exec block,
which was misleading.
BusyBox ifup in the network-multitool image needs 'address' plus
'netmask' rather than a CIDR, so split the address accordingly. Also
add 'post-up ip route replace default via <fabric-gw>' so the fabric
default overrides the management DHCP default even when one is already
installed.
