Unfriendly Ruler ring flags

Since the default RegisterFlags function is used, the help text that accompanies the flags is a bit confusing. For instance:

```
  -ruler.distributor.replication-factor int
    	The number of ingesters to write to and read from. (default 3)
```

This flag above does not have a sensible description and must be defaulted to 1 by the ruler either way.