Tezt/Cloud: use multiple DAL observers for the Echo rollup

What

When the echo-rollup follows more than a single slot, start one observer per slot and use the reverse proxy to aggregate them.

Why

Using a single observer for multiple slots will bottleneck pretty quickly, due to shard verification.

How

1. Extract the code from the etherlink operator that initializes the nodes and the reverse proxy.
2. Starts the setup with the echo-rollup when more than one slot.

The last two commits are an attempt of fixes to make nginx working with systems that don't use Sysv-init.

Manually testing the MR

Replicate the instructions from !18492 (merged), in particular you can test with a lot of producers before this MR and after, and see the result. Before the MR, you'll end up with a crashed rollup node.

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