Tezt/Cloud: Implement a default agent for running a test without VMs or to reduce the number of VMs used by a test

What

It enables any Tezt cloud test to be run by the orchestrator itself, similarly as what is done by Tezt today. This aims to work also with the proxy mode so that any tezt cloud test can run on a single VMs.

Why

Producing a slot using ghostnet via the DAL tezt-cloud scenario requires to use 3 VMs which is rather expensive if we let it run for a long time. Instead, only the proxy VM could be used in theory. This MR aims to enable this.

As a side-effect, this MR enables in theory that any Tezt test could be declared as a tezt-cloud test instead making the agent abstraction completely transparent from the test developer. While this may seem useless it could have several benefits like being able to run individually an integration test using multiple Octez versions manually.

How

We relax an agent so that it depends optionally on a point. Hence, any command given to an agent that has no point must be run locally.

Manually testing the MR

The following command should run quickly (no docker involved):

dune exec tezt/tests/cloud/main.exe -- cloud health -v --vms 0 --localhost

and the following other command should work:

dune exec tezt/tests/cloud/main.exe -- cloud dal -v --vms 0 --proxy --producers 1

with only one VM.

Finally, this should be fully retro-compatible so any command you used so far should still work.