Don't fill $SERVICE as username in service_url()

The URL generated by this plugin is invalid, but since Redis on versions 5 and lower didn't have a notion of usernames, it pretty much went unnoticed. 

Let's fix this by not setting the $SERVICE as the username in the service_url returned by this plugin.

I believe that clients ignore the username (otherwise the Redis connection wouldn't go through), so this should be a non-breaking change.

Fixes #155

functions

@@ -155,5 +155,5 @@ service_url() {
   local SERVICE="$1"
   local PASSWORD="$(service_password "$SERVICE")"
   local SERVICE_DNS_HOSTNAME="$(service_dns_hostname "$SERVICE")"
-  echo "$PLUGIN_SCHEME://$SERVICE:$PASSWORD@$SERVICE_DNS_HOSTNAME:${PLUGIN_DATASTORE_PORTS[0]}"
+  echo "$PLUGIN_SCHEME://:$PASSWORD@$SERVICE_DNS_HOSTNAME:${PLUGIN_DATASTORE_PORTS[0]}"
 }