Support for Key Generation x out of y Nodes

[SurfingNerd]

Currently the Key Generation scripts only support networks where every generated node is also an initial validator.
(Location: crates/ethcore/src/engines/hbbft/hbbft_config_generator)

This makes automation in testing very hard.
We need to be able to create testnets where we add validators later on.

Current call:

cargo run -- 8 Docker

Creates a configuration with 8 nodes, every node becomes an initial validator.
This call should be equivilant with futures call

cargo run -- 8 8 Docker

but also support the setup of having less initial validators.

cargo run -- 2 8 Docker
This call should Setup a Testnet with 2 initial Validators, 8 Nodes in Total (6 are regular nodes)

[SurfingNerd]

working branch is here: https://github.com/SurfingNerd/openethereum-3.x/tree/surfingnerd/keygen-x-of-y