isisd, yang: set default is-type to level-1-2

isisd, yang: set default is-type to level-1-2

Currently isisd has this strange (legacy) behavior where the
first area is created by default with level-1-2, while every
other subsequent area uses the default defined in the yang model,
which is level-1. This is a source of confusion when trying to
configure the daemon programatically, either with frr-reload
or using the transactional cli. Given how rare having multiple
IS-IS instances is anyway, the easiest solution is to remove
this behavior and default to level-1-2 for every area.

Note that this does not affect fabricd, which continues using
exclusively level-2-only areas.

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