tezt cloud: use static vm name

Use static names for agents, and use them for containers

How to test:

dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --proxy
dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --vms-limit 2 --proxy
dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --vms-limit 0 --proxy
dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --localhost
dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --vms-limit 2 --localhost
dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --vms-limit 0 --localhost

dune exec tezt/tests/cloud/main.exe -- dal cloud --bootstrap -v --observer-slot-indices 1,2 --producer-slot-indices 3,4 --localhost

And check there was no regression

A little change that was introduced is also to use the agent names as container names in localhost