Fixes #8727 - DHCP validation does not fail on discovered leases

For description read the conversation on the ticket.

DO NOT MERGE.

Greg it looks like this does not work. I might have a mistake in my tests tho,
but I still see `127.0.0.1` as a conflict in the test.
```lease1``` and all related stubs can be put into a setup block of a context these two tests can share to avoid having the exact same code in two tests.