err if environment vars controller conflicts

[Anastasia]

It is possible to specify what controller model commands will use via $$JUJU_CONTROLLER=<controller name> and $JUJU_MODEL=<controller name>:<model name> environment variables.

If commands do not have a controller name explicitly stated as part of either '-m <controller name>:<controller model>' or '-c <controller name>' but there are both environment variables present and their controller name differ, we should err out until user clarifies what is needed.

[Anastasia (anastasia-macmood)]

PR against 2.5: https://github.com/juju/juju/pull/9804