Update dhcp host files that we don't manage anymore

If, for some reason, a node has less MAC addresses than before, we need
to drop the DHCP host files that we won't update with chef anymore.

As the host files have a format like
"d52-54-00-21-c3-72.site-$index.conf", we simply remove the host files
where the index is too high.
Pass to next node earlier if the state doesn't require an update

This helps make sure that everything we do after this test is actually
useful and won't affect a node while it shouldn't impact it.
Do the state update for a node even if no boot IP is known