Constraints should allow selection of other parameters

We have some OpenStack flavors that differ only by the presence or absence of a non-root ephemeral disk. We need some way of directing juju to select between them when creating machines.

http://juju-docs.readthedocs.org/en/latest/provider-configuration-openstack.html mentions a setting called default-instance-type which mapped directly to an OpenStack flavor. (Not sure why it wasn't called default-flavor...) I have been unable to test yet whether default-instance-type is still supported, but it does not appear in a search of http://juju.ubuntu.com/. (It's unclear to me which of these sites is more authoritative.)

So, if default-instance-type is still supported:
- it needs documenting
- it should be possible to override it in constraints with "instance-type=xyz"

If default-instance-type is no longer supported:
- OpenStack flavors should be selectable as constraints using "openstack-flavor=xyz" or similar (preferred), or
- Other constraints should be provided to enable selecting non-root disks or lack thereof (non-preferred).