Check loopback and link-local multicast endpoints

Previously we just disallowed link-local (unicast).  This disallows loopback
and link-local multicast.

pkg/master/master.go

@@ -286,7 +286,7 @@ func setDefaults(c *Config) {
 	if c.CacheTimeout == 0 {
 		c.CacheTimeout = 5 * time.Second
 	}
-	for c.PublicAddress == nil || c.PublicAddress.IsUnspecified() {
+	for c.PublicAddress == nil || c.PublicAddress.IsUnspecified() || c.PublicAddress.IsLoopback() {
 		// TODO: This should be done in the caller and just require a
 		// valid value to be passed in.
 		hostIP, err := util.ChooseHostInterface()