charm: avoid use of /bin/cp

Isolated tests should avoid use of external commands when possible.

Also update utils dependency so that we can use the new fs package
containing the Copy function.
provider/openstack: support zone placement
charm/testing: do not rely on copy overwriting
Merge pull request #37 from axw/provider-openstack-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.
Merge pull request #57 from rogpeppe/006-charm-no-cp

Isolated tests should avoid use of external commands when possible.

Also update utils dependency so that we can use the new fs package
containing the Copy function.