Use UUIDs as vm name

What is covered in this MR:

• change default behavior of machine name, from now uuid is send to vm-bhyve as machine name and the machine name is send as a hostname
• name now can be longer
• initial test coverage for machines

Warning: Machines created with old naming convention <team_id>_<machine_name> won't be visible for the runhyve unless the uuid column will be erased.

You can use empty all uuids with psql:

UPDATE machines SET uuid = NULL;

or iex -S mix:

Webapp.Repo.update_all(Webapp.Machines.Machine, set: [uuid: nil])