Comment 10 for bug 1425808

Part of this issue is resolved with: https://github.com/juju/juju/pull/7300

It will allow the openstack provider to look at the fault message while the instance is in
an error state to determine if error was caused by 'No valid host'. It was tested in an openstack
producing the No valid host message, however there were not multiple AZ to complete testing.

There is existing code to try another AZ if the current one tried fails with 'No valid host'.