Add the upgrade step, tested manually, but needs real tests.
Tests for the upgrade step.
Tweaks.
update error message
Add a comment.
Merge pull request #1185 from howbazaar/1.20-fix-juju-run

Add the upgrade step, tested manually, but needs real tests.

Create a new system identity for those systems where it has been removed.

(Review request: http://reviews.vapour.ws/r/495/)