1.21 critical issue - fixing juju-run

[Tim Penhey]

Hi Curtis,

https://bugs.launchpad.net/juju-core/+bug/1392745

This needs to be fixed on 1.20, 1.21 and master branches.

The fix has landed on 1.20.
Hopefully landed on 1.21 by the time you read this.

Work needed to tweak for master due to different ways the upgrade steps
are being run for now.

I think this is a fairly low risk change for 1.21, and easily checked
with the standard upgrade tests and a check of 'juju run' after the
upgrade has completed.

Cheers,
Tim