Make IP prefix configurable

This commit makes the IP prefix used to generate addresses configurable
to users. This can be useful if you would like to use a smaller range or
if your current setup is overlapping with the current range.

The current range is left as a default

app_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"gopkg.in/check.v1"
+	"inet.af/netaddr"
 )
 
 func Test(t *testing.T) {
@@ -36,7 +37,9 @@ func (s *Suite) ResetDB(c *check.C) {
 	if err != nil {
 		c.Fatal(err)
 	}
-	cfg := Config{}
+	cfg := Config{
+		IPPrefix: netaddr.MustParseIPPrefix("127.0.0.1/32"),
+	}
 
 	h = Headscale{
 		cfg:      cfg,