Handle localhost distributed setups properly (#8577)

Fixes an issue reported by @klauspost and @vadmeste

This PR also allows users to expand their clusters
from single node XL deployment to distributed mode.

pkg/net/host.go

@@ -81,9 +81,12 @@ func (host *Host) UnmarshalJSON(data []byte) (err error) {
 
 // ParseHost - parses string into Host
 func ParseHost(s string) (*Host, error) {
+	if s == "" {
+		return nil, errors.New("invalid argument")
+	}
 	isValidHost := func(host string) bool {
 		if host == "" {
-			return false
+			return true
 		}
 
 		if ip := net.ParseIP(host); ip != nil {