ospfd: fix output of dead-interval in show running

ospfd: fix output of dead-interval in show running

When you set OSPF hello-interval for an interface and dead-interval is
not set for this interface, dead-interval will be calculated and set
automatically. "show running-config" will contain an invalid command:

    test(config)# interface vpp1
    test(config-if)# ip ospf area 0
    test(config-if)# ip ospf hello-interval 1
    test(config-if)# exit
    test(config)#
    test(config)# do show running-config
    ...
    interface if1
     ip ospf area 0
     ip ospf dead-interval minimal hello-multiplier 0
     ip ospf hello-interval 1
    !
    ...

It causes frr-reload.py to fail because of this:

    # vtysh -c "show running-config no-header" | vtysh -m -f -
    line 9: % Unknown command:  ip ospf dead-interval minimal hello-multiplier 0
    ...

With this change, output "ip ospf dead-interval" only if it has value
configured explicitly.

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