Env cmd manual

This work removes the dependency that the manual provisioning had on the environment name.

Instead, the provisioning requires something that supports the API it needs to call.  This means that the interface that the add-machine command depends on needs a few more methods.

The AddMachines client method is now used, and the 1.18 fall-back code is removed.
I'm not removing it from the API client, just the use of it from the juju client.