ospfd: install Type-7 when NSSA area is configured after redistribution

ospfd: install Type-7 when NSSA area is configured after redistribution

Currently, if NSSA area is configured before redistribution is enabled,
Type-7 LSA's are installed and flooded. But if NSSA area is configured
after redistribution is enabled, Type-7 LSA's are not installed.

With this change, when NSSA area is configured, schedule a task that
scans for external LSA's. If they exist, install Type-7 and flood to
all NSSA Areas.

There already was an attempt to fix this problem in 0f321812f where
ospf_asbr_nssa_redist_task() was triggered in ospf_abr_task_timer().
This turns out to be incorrect place for this operation because it's
a one-off operation needed only after "area <ID> nssa" execution. And
ospf_abr_task_timer() is a periodic operation. Triggering
ospf_asbr_nssa_redist_task() in ospf_abr_task_timer() caused a problem
that was fixed in 945eec2b6 making the problem with NSSA area
configured after redistribution actual again.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>