openstack plugin: Change METADATA_URL

This makes things work on the openstack install I'm using.
I didn't had a deep look at `ohai/mixin/ec2_metadata` which has a `best_api_version` method, I can provide you with some debugging. I can also add a `set_debug_output`on Net::HTTP stuff somewhere ...
I'm also wondering if we shouldn't use "1.0" rather than "latest" which my be subject to schema changes.