Tezt_cloud: Node_helpers: Refactor code

What

This MR is meant to simplify the logic in the node_helpers.ml module, and therefore make it more usable by tezt-cloud experiments. I believe the module was very repetitive and error prone.

Why

How

Manually testing the MR

Use you favourite DAL and LAYER1 scenarios, as this MR should be a no-op.

What I used and they worked fine:

dune exec tezt/tests/cloud/main.exe -- LAYER1 -v \
  --stake 1,1 \
  --network ghostnet \
  --faketime "-$(( $(date --utc +%s) - $(date --utc --date $(./octez-node snapshot info --json ./ghostnet.snapshot | jq -r .snapshot_header.timestamp) +%s) ))" \
  --tezt-cloud debian \
  --snapshot file:./ghostnet.snapshot \
  --localhost

and

dune exec tezt/tests/cloud/main.exe -- DAL --producers 2 --network ghostnet --tezt-cloud dal -v --snapshot file:./ghostnet.snapshot --localhost --fundraiser edsk3nd3m5Zb31fbchqAvWVUbeF4jH7kdcs6mRCA7rFzYt4kopSyZP --stake 1,1

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR