This commit reworks getAvailableIp with a "simpler" version that will
look for the first available IP address in our IP Prefix.
There is a couple of ideas behind this:
* Make the host IPs reasonably predictable and in within similar
subnets, which should simplify ACLs for subnets
* The code is not random, but deterministic so we can have tests
* The code is a bit more understandable (no bit shift magic)
