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)

app_test.go

@@ -38,7 +38,7 @@ func (s *Suite) ResetDB(c *check.C) {
 		c.Fatal(err)
 	}
 	cfg := Config{
-		IPPrefix: netaddr.MustParseIPPrefix("127.0.0.1/32"),
+		IPPrefix: netaddr.MustParseIPPrefix("10.27.0.0/23"),
 	}
 
 	h = Headscale{