show-controller should indicate that it requires superuser access

[Cory Johns]

The show-controller command seems like it should would give you similar info as from `juju list-controllers --format=yaml` but instead gives you all of the more detailed info if you are a superuser, or nothing if you are not.

It should either inform you that you don't have sufficient permissions and point to the list-controllers command, or give you a subset of info appropriate to your level of permissions.

[Anastasia (anastasia-macmood)]

By design, the intention of all show-<something> provides more detailed information about <something> than list-<somethings>.

We should be more explicit about what information to expect from show-controller for each access level in command help.

[Canonical Juju QA Bot (juju-qa-bot)]

This Medium-priority bug has not been updated in 60 days, so we're marking it Low importance. If you believe this is incorrect, please update the importance.