isisd: retrofit 'router isis' and 'ip router isis' cmds

isisd: retrofit 'router isis' and 'ip router isis' cmds

These are complex commands to retrofit, partly due to the number of
different callbacks they touch. Additionally, in FRR adding
an interface to an IS-IS area that does not exist also creates that
area. To make sure that this behavior is kept, while at the same
time keeping the northbound api consistent, we need to take extra
care to call the appropriate callbacks to update the YANG tree.

Note that many callbacks rely on the existence of the corresponding
IS-IS area; when these callbacks are joined together in a single
transaction, we need to ensure that the area creation is performed
first, or the config will fail. For this reason, the isis instance
create callback has been given a slightly lower priority than the
others.

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