Add a private testnet module

[jaredponn]

This PR:

• Adds a private testnet module which reuses the node.nix module which includes
  • a CLI utility -- request-from-faucet -- which acts as a testnet faucet
  • an initial fund distribution provided by Nix which gets distributed from the faucet by a oneshot systemd unit
  • tests to verify that this "works as expected"
• generalizes some of the settings in the node.nix module (these changes are backwards compatible) to accommodate the issues with setting up a private test net
  • provides an option to -- at nix eval time -- copy the config.json to /etc/cardano-node which is true by default (for backwards compatbility)
  • sets the CARDANO_NODE_NETWORK_ID environment variable similarly to how the CARDANO_NODE_SOCKET_PATH environment variable is set

Example usage:

https://github.com/mlabs-haskell/cardano.nix/blob/c3d804962846176068cfffad2ea3e2796b689164/tests/test-node.nix

It pretty much amounts to just enabling the test-node.enable option, and setting the test-node.initialFunds = { <addr> = <amount>;} and it'll do the rest for you. It really is a drop in replacement!

Notes:

• I honestly wasn't sure if I did the licensing things correctly i.e., the reuse check. So I welcome feedback with that :) or anything else that seems amiss

[aciceri]

Great job, we already wanted something like!

I added the following:

• removed all the license files by adding a new entry to the .reuse5/deps file
• renamed your test (and also the old cardano-node one)
• rephrased your comments into a single one making a bit more synthetic (keeping only info interesting for a potential reader)

Since the test seems to work looking at logs I'm merging it but I've one doubt: how does it since the Producers list is empty in the topology file? Nevertheless I see that blocks seem to be produced in logs.

[aciceri]

I've just renamed other files to make names more clear IMO.
I've also tried adding your new module to the autogenerated docs in the website but then I changed my mind, after all is not a module supposed to be used by other people, it's more a sort of helper module that is going to be used only in a specific test.