tools: Fix frr-reload for ebgp-multihop TTL reconfiguration. (backport #17946)

[mergify]

In ebgp-multihop, there is a difference in reload behavior when TTL is unspecified (meaning default 255) and when 255 is explicitly specified. For example, when reloading with 'neighbor ebgp-multihop 255' in the config, the following difference is created. This commit fixes that.

Lines To Delete
===============
router bgp 65001
 no neighbor 10.0.0.4 ebgp-multihop
exit

Lines To Add
============
router bgp 65001
 neighbor 10.0.0.4 ebgp-multihop 255
exit

The commit 767aaa3 is not sufficient and frr-reload needs to be fixed to handle both unspecified and specified cases.

---

How to reproduce the issue on master branch

cat <<EOF > /etc/frr/frr.conf
router bgp 65001
 bgp router-id 10.0.0.1
 neighbor 10.0.0.4 remote-as 65002
 neighbor 10.0.0.4 ebgp-multihop 255
exit
!
end
EOF

systemctl start frr
./tools/frr-reload.py --debug --test /etc/frr/frr.conf

These commands above shows the diff as below:

Lines To Delete
===============
router bgp 65001
 no neighbor 10.0.0.4 ebgp-multihop
exit

Lines To Add
============
router bgp 65001
 neighbor 10.0.0.4 ebgp-multihop 255
exit
```<hr>This is an automatic backport of pull request #17946 done by [Mergify](https://mergify.com).