[Feature Request] Warn users about deploying an application on the controller model

[Brandon Castillo]

Because controller models cannot be migrated it would be a good idea to add a warning. Typically for most use cases applications should not be deployed on the controller model. A warning prompted to the user asking if they're sure they want to deploy an application to the controller model may be useful for new users and avoid potential issues if a user wishes to migrate to a newer version of juju in the future.

Example when bootstrapping:
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Bootstrap complete, controller "<controller-name>" is now available
Controller machines are in the "controller" model
WARN: It is not recommended to deploy any applications in the "controller" model

Now you can run
 juju add-model <model-name>
to create a new model to deploy workloads.
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Example when deploying:
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juju deploy <application>
WARN: It is recommended to deploy applications in another model. Are you sure you want to deploy <application> in the "controller" model? y/N
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