Machine placement doesn't support existing machines

[Jorge Niedbalski]

[Rationale]

There are scenarios (such as MAAS deployments ) on which you have pre-allocated machines
with custom settings.

Currently using the v3 bundles juju-deployer doesn't allows to place a service on a machine different than 0 or specify a new container for deploy this machine.

Since the introduction of v4 bundles ( already supported on juju-deployer ) is possible to place
a service into a specific machine, container or even create a new lxc container.

However, it forces the creation of new machines or containers. The specific case on which you already have a deployed machine, and you don't want to create/allocate a new one for the service is not covered.

This bug tracks that functionality.