provider/azure: simplify role size selection, report hardware, fix costs

- Use the newly exported environs/instances.MatchingInstanceTypes
  to simplify instance type selection when force-image-name is used.
- Update role costs and role sizes (bump gwacl rev).
- Emit hardware characteristics by converting gwacl.RoleSize to
  instance.HardwareCharacteristics.

Fixes https://bugs.launchpad.net/juju-core/+bug/1215177
If there's only AMD64 then arch and instance-type constraints should be made to conflict