Fee voting settings from the configuration file

@tdfischer, @sublimator 
```
Communicating Fees to Clients
If the base fee differs from the network default, it is included in the ledger. The fee schedule is always communicated to the client when it subscribes to the ledger stream.

For base fees, four values are included in the JSON, all integers:

fee_ref
The number of fee units the reference transaction costs.
fee_base
The XRP cost of the reference transaction.
```

I guess it could just as well be called `reference_fee` but we already have `fee_ref` and `fee_base` so it's potentially confusing. 

It seems also later, we hard code the `fee_ref` as 10 in the voting transaction.

```c++
        SerializedTransaction trans (ttFEE);
        trans.setFieldAccount (sfAccount, Account ());
        trans.setFieldU64 (sfBaseFee, baseFee);
        trans.setFieldU32 (sfReferenceFeeUnits, 10);
        trans.setFieldU32 (sfReserveBase, baseReserve);
        trans.setFieldU32 (sfReserveIncrement, incReserve);

```

Do we feel that a setting for `fee_ref` adds no real value?