isisd, yang: set default network-type to broadcast

isisd, yang: set default network-type to broadcast

isisd has both a circ_type and a circ_type_config variable to track
the network tpye of an interface. The former has no default, but the
latter defaults to broadcast. Adding that default makes sure that the
yang leaf won't be deleted, which is something that would not make
sense from an isisd perspective. We will need to add an operational
state leaf to match the potential difference between the configured
network type and the actual network type, since the latter might be
different based on the interface flags received from zebra.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>