Need a provider type which provisions via SSH
For environments where there is a lot of standing resources, or a desire to do the provisioning in a way juju won't support, it would make sense to have a provider which just installs juju onto existing servers as its "start_machine" step.
* Static list of servers fed into ZK and/or maintained on the provisioning agent system.
* SSH to install agents
* Containers or chroots so charms can still run with assumption of a clean system.