provider/openstack: implement AZ spread

OpenStack counterpart to #55

Fixes https://bugs.launchpad.net/juju-core/+bug/1183831
Merge pull request #70 from axw/provider-openstack-zone-spread

provider/openstack: implement AZ spread

OpenStack counterpart to #55

Fixes https://bugs.launchpad.net/juju-core/+bug/1183831