provider/openstack: support zone placement

This is essentially the same as the work done for ec2 in https://github.com/juju/juju/pull/8. Like many other things in the two providers, there is a lot of overlap.

I have not yet updated dependencies.tsv; will do so when the goose branch lands (https://codereview.appspot.com/103900045).

A followup will add support for automatic spread across AZs.