etcdserver: stop using addScheme

This standardizes the flags to use a list of URLs everywhere. The next step
is to enforce the scheme based on TLS settings and support compat flags.
This function can be simplified to:

```
all := make([]string, len(*us))
for i, u := range *us {
  all[i] = u.String()
}
return strings.Join(all, ",")
```