Cardano-node: Make `cardano-node` slot length derived from genesis

[deepfire]

Subtask of #255

Slot length is currently hard-coded in ourobboros-network to 20s [1], which is counterproductive for short CI runs, that would benefit from 1s slots.

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1. https://github.com/input-output-hk/ouroboros-network/blob/master/ouroboros-consensus/src/Ouroboros/Consensus/Node.hs#L188

[Jimbo4350]

@deepfire My branch jordan/replace-cardano-configuration has a key in the soon to be config yaml file to set the slot length.

[dcoutts]

The slot length is already specified in the genesis file. It should be taken from there, not hard coded. But it should not be a config file or CLI item. It's critical for consensus that everyone agree on the slot length, so it is right that it comes from the genesis.

[deepfire]

Issue moved to input-output-hk/ouroboros-network #1344 via ZenHub