Rework getAvailableIp

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)

cli_test.go

@@ -15,6 +15,7 @@ func (s *Suite) TestRegisterMachine(c *check.C) {
 		DiscoKey:    "faa",
 		Name:        "testmachine",
 		NamespaceID: n.ID,
+		IPAddress:   "10.0.0.1",
 	}
 	h.db.Save(&m)