Improve usability of the migrate command

[alejdg]

In the migrate your model doc it is stated:

The following command will migrate a model named my-model to the desired controller, in this case called my-controller.

MODEL_NAME=my-model
MODEL_UUID=$(juju show-model $MODEL_NAME --format yaml | yq .$MODEL_NAME.model-uuid)
jimmctl migrate my-controller $MODEL_UUID

The user shouldn't need to run a juju command to get the the model UID to use that in a jimmctl command. To improve the usability, jimmctl should be able to do jimmctl migrate my-controller my-model.

If there's ambiguity in the my-model name since there could be models with the same name but in different controllers, jimm should require a unique identifier like ..