Comment 2 for bug 1564622

Copying from dup'ed bug for additional background and suggested wording:

When a Juju 1.X user is dist-upgrading to xenial and uses Juju 2.0 for the first time, Juju should help the user along with the change in 1.X -> 2.X by providing a one time message hint that Juju 1 is still available at a new location for them to continue using.

An example user flow:

    do-release-upgrade
    .... lots of things

    juju switch
    Welcome to Juju 2.0. You can continue using juju-1.25.5 with the command juju-1 e.g. `juju-1 switch`

It should detect this situation if the user has existing juju 1.X config but no juju 2.X config. It should also only be run on the first occasion so that we don't spam the user over and over.