Calculate ForgeBatch gasLimit with parametrized formula

Add the following config parameters at
`Coordinator.EthClient.ForgeBatchGasCost`:
- `Fixed`
- `L1UserTx`
- `L1CoordTx`
- `L2Tx`
Which are the costs associated to a ForgeBatch transaction, split
into different parts to be used in the formula to compute the gasLimit.

cli/node/cfg.buidler.toml

@@ -104,6 +104,12 @@ GasPriceIncPerc = 10
 Path = "/tmp/iden3-test/hermez/ethkeystore"
 Password = "yourpasswordhere"
 
+[Coordinator.EthClient.ForgeBatchGasCost]
+Fixed = 500000
+L1UserTx = 8000
+L1CoordTx = 9000
+L2Tx = 1
+
 [Coordinator.API]
 Coordinator = true